The stuff that makes your pee go yellow is a chemical called urobilinogen.  This is a breakdown product of the haemoglobin that makes your red blood cells red.  When you have red blood cells, they’re made in the bone marrow, you have millions and millions of them and you destroy something like 10^11 (100,000,000,000) of these cells every day.

When you break down the red blood cells, which have a lifespan of about 120 days, the haemoglobin molecule, responsible for binding up the oxygen and oxygenating your tissues, gets split open by chemicals called enzymes.  There’s a little atom of iron sat in the middle of a haemoglobin molecule. This iron is released and can be recycled in the body but the coloured molecule that used to wrap up the iron, the haemoglobin molecule, has to be broken down even further.

Once you’ve unwrapped the iron you have this molecule called biliverdin which is green.  It then gets oxidised a bit and becomes something called bilirubin which is a brown or yellow colour.  This floats around in the bloodstream and gets picked up by the liver which adds a couple more chemicals to it to make it dissolve in water.  The liver then squirts it out in your bile. That’s why bile is yellow [which you’ll see] if you’re sick: people who are having norovirus problems at the moment, 3,000,000 of them in the UK know about that only too well. The bile goes into your intestines, which are rich in bacteria.  Bacteria in the intestines chop up the extra atoms and chemicals that were added to the sides of the bilirubin molecule and knock some of them off, turning it into something called urobilinogen.  The urobilinogen is still yellow but it does dissolve in water.  This gets reabsorbed into your intestines and some of it ends up in your bloodstream.  Because it goes into water then when the blood goes through the kidneys it ends up in your urine and it comes out as a yellow colour. So that’s why you have yellow urine!

Some of it carries on in your intestines and bacteria change it a bit more and it turns into another molecule called stercobilin and that’s brown. That’s why you do brown poos!

We answered this along with Bill Gettys' question...

When lightning hits water, it suddenly dumps a huge amount of energy into the bit of water it’s hitting – so very close to where it’s hit it will form steam.  When lightning hits sand it releases so much energy it causes sand to melt and turn into glass.  It would probably vapourise the water near where it’s hit and cause a bubble of steam which probably will cause a splash.

If you’re swimming and lightning hits a lake it is actually very, very dangerous.  If lightning hits the middle of a lake water has got some salts in it so it will conduct electricity a bit.  You would conduct electricity better.  So if you’re swimming in a lake and there’s a big current from the lightning strike flowing through the lake it sees you as a kind of ‘short’ as it can jump 2m to an easy path.  So it will flow through you.  You’d get a big current flowing through you which would be very dangerous.  It can stop your heart which is why I was told in the States that you don’t go swimming in a thunderstorm.

The fish could end up fried as well then!

We answered this along with Robert Sullivan's question...

When lightning hits water, it suddenly dumps a huge amount of energy into the bit of water it’s hitting – so very close to where it’s hit it will form steam.  When lightning hits sand it releases so much energy it causes sand to melt and turn into glass.  It would probably vapourise the water near where it’s hit and cause a bubble of steam which probably will cause a splash.

If you’re swimming and lightning hits a lake it is actually very, very dangerous.  If lightning hits the middle of a lake water has got some salts in it so it will conduct electricity a bit.  You would conduct electricity better.  So if you’re swimming in a lake and there’s a big current from the lightning strike flowing through the lake it sees you as a kind of ‘short’ as it can jump 2m to an easy path.  So it will flow through you.  You’d get a big current flowing through you which would be very dangerous.  It can stop your heart which is why I was told in the States that you don’t go swimming in a thunderstorm.

The fish could end up fried as well then!

The reason you get joint cracking is because there is fluid inside your joints.  When you move a joint the pressure in this fluid drops and this can cause a bubble to form made up of the gas dissolved in the liquid.  The bubble pops into existence and it will take up about 15% of the space inside the joint.  This causes all the little ligaments and supporting structures around the joints to pop outwards, so that’s the first crack you hear.  
When you carry on moving the joint it goes click again as the bubble disappears.  This happens when you put the pressure back up in the joint and it the bubble collapses on itself.  That releases even more energy and that’s the second sound that you hear.  It only releases about 5% of the energy that would be able to damage cartilage, so there’s no link to arthritis if you crack your joints. Although there were some interesting studies done in Germany, where they looked at someone who cracked the knuckles of their left hand for 35 years but never those of their right hand.  There was no difference in arthritis but the muscles on the left hand were much weaker than the muscles on the right hand. Explain that one, if you will.

The picture shows a plane flying through the sky with the usual contrail, the exhaust fumes of the plane, and extending in front of the plane is a very long black, thin line on the picture.

I think that the sun is behind the plane and casting a shadow of the contrail. As the sun is behind the plane the shadow appears in front of the plane, as it were.  The cloud is so wispy and tenuous the only way you can see it is because the sun’s lighting it up.  Once you’re in shadow you can’t see the cloud so it looks like the cloud’s disappeared.  Actually, if you look very closely above the contrail you can see a little bit of a shadow following above the contrail as well. I think what it is, it’s just a shadow cast from an interesting direction.

What could be happening is the fact that you are blocking the sound that you can hear when you put your finger in your good ear suggests that your good ear is working perfectly and it’s working perfectly for the normal way in which sounds are conducted into the ears: Vibrations in the air go onto your ear drum, make your ear drum move and that’s turned into electrical signals in your inner ear that your brain can understand.

Your hearing aid on the other side ought to be just working by amplifying the sound that you’re hearing and making bigger movements in the ear on that side so that the signals that go into the inner ear and get converted into electrical signals. Even though the ear doesn’t work as well, the signals from the hearing aid are so much louder it allows what hearing is there to work a bit better.  
The fact that your hearing is lost when you put your finger into the other ear and block off the conduction of the sound suggests that the hearing aid is just not succeeding on that side in managing to make what’s left of your hearing there actually register anything that you can physically hear.  I think that’s the problem. I don’t think the hearing aid’s faulty necessarily. If it’s been checked, it’s been checked. I think it’s possibly that your ear’s not working as well as it could.

There are various therapies that are in the pipeline now to help people who’ve lost the ability of the inner ear to detect sound and turn it into electrical signals.  What scientists are now exploring is the idea of stimulating the nerves that connect the inner ear directly to the brain and you can get quite good discrimination of sounds that way.  A bit like a cochlear implant but a bit more advanced.

The answer is it’s all down to concentration.  The more you drink the more dilute your urine is and therefore the more dilute any colourants (the urobilinogen) that’s being made and filtered from the bloodstream into it.  If you haven’t drunk very much you continue to break down red blood cells and produce this urobilinogen and therefore you will have urine which tends to be a darker colour.  Conversely if you drink a lot, it’ll dilute it and the urine goes a lighter colour.  There is an exception to this which is that if you have a problem with the bile flowing out of the liver because you have gallstones, for example, then the bilirubin that is the thing that produces this urobilinogen in the first place can’t get out of the body and therefore you have a build-up in the bloodstream and this can end up producing a dark urine.  You get very, very dark urine under those circumstances.

The thing that makes that happen is protein.  Sometimes, if you have certain problems with the kidneys you can produce very frothy urine: it’s almost like champagne.  You get this massive head on the urine.  That’s a serious problem but the day-to-day occasional frothiness is just because occasionally there’s more protein in your urine than others.  The protein breaks down the surface tension of water and you get these bubbles forming.  It acts a bit like bubble bath which is why you get the fizz.

If you take a proton or something and make it go really, really fast – give it lots of kinetic energy then relativity says it gets heavier.  In the same way if you’ve got lots of potential energy if you put it into a really big heavy atom like uranium then it will also get heavier. When you split up that big atom you get less potential energy and that means that the resultants (all of the protons and all of the neutrons inside) are lighter than they were before because they have less potential energy.

So there are actually the same number of constituents, but the sum total weight is decreased.

Yes, you do.  Although your mass is the same so that you’ve got the same amount of fat, the same amount of you.  When you’re at the equator you’re spinning round so you’re getting thrown away due to the centrifugal force (the same force if you’ve ever sat on a roundabout and it spinning around you can feel yourself being pushed away from the centre).  You’re actually even lighter than even the centrifugal force would give the impression of because the Earth actually bulges out at the equator so you’re further from the centre of the Earth.  You’re actually even lighter even if the Earth wasn’t spinning than you’d be at the poles.

We calculated it’s something like 0.48g lighter for a 100kg person. They’re gonna be about 0.5g lighter when the moon is directly overhead because the moon is attracting you in the same way it attracts water when it makes tides.

Saliva is made in the same way that you make tears.  You have glands which have a rich blood supply.  When the blood goes through it passes through very thin walled capillaries which are a little bit leaky.  It’s exactly the same as if you go to the garden centre and you see those watering systems where the pipes have small holes in.  You turn the tap on and the water going through squeezes some of the water out of the pipe.  Why do you make saliva, in other words watery liquid and not blood?  The answer is the walls of these tiny capillaries are just big enough for water molecules but they’re too small to let the bigger protein and cellular components of blood leach out.  Some things can get out: the water and some antibodies but that’s about it.

Saliva also stops viruses because it contains antibodies.  It’s got an antibody called IgA which is one of the immunoglobulins which can neutralise viruses.  Yes you can mop up a lot of colds and viruses and bacteria with the constituents of saliva.  The antibody is secreted - you actually have a pump which pushes them into the saliva.  They’re added after you’ve made the saliva or tears.