Everyone’s noticed this onion effect and people just think ‘oh, there’s something in the onions’ but what’s interesting is next time you’re cutting onions, time how long it takes between cutting into the onion (and the spray you can see coming up under the knife) and the time it takes before your eyes start reacting. 

You’ll see there’s a big delay between the onion being chopped up and you feeling the sting.  The reason is there’s a chemical reaction going on the minute you start breaking into the onion, that starts to trigger the eye smarting effect. 

There are enzymes locked away in the onion called alinases and they’re names after the family you find them in, like onions, garlic, scully and chive.  They’re the alium family, that’s why it’s called alinase.  This breaks down a family of chemicals, which are also in the cells called amino acid sulphoxides.  They’re the smelly things that make onion smell like onion. 

As the enzyme starts chewing into them, it releases something called sulphinic acid and sulphinic acid then breaks down into another chemical which is called sin-propantheal-S-oxide and that’s the irritant.  It comes squirting out of the onion towards your eyes, binds to the corner of your eyes, the cornea, which is very rich in nerve supply (it’s got some of the densest nerve fibres in the body) which signal pain and this is an irritant.  These nerve fibres are wired up to your lacrimal gland which makes tears, because your eye correctly says if there’s something irritating it there’s probably a foreign body in the eye, therefore the way to get rid of it is to activate more tears and wash it out. That’s why you cry.

But why does it take time before that kicks in? because that chemical reaction has to happen? Why does a contact lense make a difference? Well, a contact lense sits on the front of your eye, in front of the cornea where there’s the most nerves, it stops the chemical getting into contact with the nerves and instead, because contact lense wearers probably make more tears as they’ve got a foreign body in their eye all the time, it’s washing the stuff off the contact lense and down your tear duct before it has a chance to irritate the nerves at the front of the eye.

Dave: I think it probably is. When water turns from a liquid into a gas, it needs lots and lots of energy, that’s why if you sweat on a windy day you cool down because the water evaporated off your skin which absorbs lots of energy and cools you down. Essentially that is what the wet towel is doing. It’s evaporating so that absorbing lots of energy, cooling everything down. Ideally you wants it to by slightly windy, you probably don’t want too much sun on the outside as it will be heating it up. Unless, the sun heating it up is causing air currents, it’s making its own winds because it gets hot and air expands so it floats upwards.

Chris: I did try this when I was in Boston, It was so hot and I was in this room, and they said they would let me borrow a fan from reception for $5 so I took this fan up to my room. I then though all this is doing is blowing hot air from outside into my room, and making it hotter. So I thought I’d apply a bit of physics. I went and got a towel, dunked it in the sink, making it really wet and draped it over the fan. For a while it worked beautifully. The cold air coming through was evaporating water off of the towel, bringing down the temperature of the air going through and cooling my room down! That is until the fan blew up and filled the room with smoke because it had gotten water in it!

What’s happening when the sun is on your skins is that its causing damage in your skin. The Ultra Violet radiation on from the sun is zooming into your skin cells and causing the DNA in your cells to be damaged. This is prompting a lot of emergency reactions to go on in your skin cells. They realise they’re damaged and lots of gene expression and signalling pathways get switched on in response to this damage to either patch up the cells that have been damaged by the sun or to kill themselves to protect your body from this damage. This actually takes some time to get going.

At the same time, because you’ve damaged your skin, there’s going to be some kind of immune response going on as well, your body’s activating itself to the damage so this is why you’re going to go red, get inflammation, and in some really serious cases you’ll get skin blistering and swelling but this does all take time. These are biological processes, the sensing of damage is quite quick but actually getting the response going might well take a couple of hours. It’s best not to get sunburnt if you want to reduce your risk of skin cancer.

There are a few things contributing to this, firstly, a melon is roughly spherical, and a sphere has a low surface area for it's volume.  For the melon to heat up, the sun must shine on its surface, and so in this situation, there's not much surface conpared to the volume of the melon.  Also, a melon contains a lot of water, which has a high 'specific heat'.  This means that you need to put lots of energy in to raise the temperature just a little.  Even if the water does heat up, the flesh of the melon will not let the water move around inside, so the heat is not spread through, but needs to pass from one cell to the next.

It's also possible that the melon loses water by evaporation, which would also help to cool it down.

Batteries can heat up if you have a short circuit.  Instead of the electricity going through a circuit where it is used up in various ways or resisted, it just goes straight through the battery, and is then conducted back around into the battery again.  All of the energy from the battery is released as heat in the battery, and it can get dangerously hot.

We think that an ant is so small that it's terminal velocity, the fastest it will travel in free fall through air, doesn't actually get fast enough to kill it.  So in theory, you could drop an ant from any height and it would still be okay.

Urine will contain lots of things that plants like such as water and nitrates, which can act like fertilizers.  But if you wee in the same place too much, you will build up a concentration of salts in the soil, and this can effect the osmosis of water into the plant's roots.

This is called the rhesus effect.  The rhesus gene adds an extra 'label' to the surface of your red blood cells, like the labels for group A or group B, this is type D.  If you are rhesus negative as a mother, it means you will not have any antibodies to this label in your blood, as your immune system has never seen it.  If your partner is rhesus positive there is a chance that a baby you conceive will be rhesus positive too.
When the baby is born, blood from the baby can get into the mother's blood stream, and so the mother makes antibodies against the baby's blood.  This wont harm the baby this time round, but may cause complications during the next pregancy.
Mothers protect their baby from 30 weeks by putting a cross section of their antibodies into the baby's circulation.  This will now include the "anti-D" antibodies, so if the baby is rhesus positive these antibodies will lock onto their blood cells and start to destroy them.  This can mean the baby becomes anaemic inside the womb, and so you either need to induce birth quickly or give the baby a blood transfusion inside the uterus.  To prevent this happening in the first place, rhesus negative women are given a dose of Anti-D antibodies, up to 72 hours after her first baby if they couldn't determine the baby's blood type.  The anti-D antibodies soak up any of the baby's blood so that the mother's immune system doesn't know how to make these antibodies.

Some bacteria are really tough.  They have protective layers around them that enable them to survive acid.  Some viruses actually need the acidic environment of your gut in order to infect you.  Some bacteria, such as Clostridium perfringens which causes gas gangrene and Clostridium dificile (C.dif) form spores.  These spores are tiny husks of dried up bacterium, almost in suspended animation, which can pass through the stomach without being damaged and they can come back to life in the relatively nice environment of the intestines.

Other bacteria, like Helicobacter pylori, which is linked to causing stomach ulcers and stomach cancer, encode an enzyme called uricase.  Uricase breaks down urea which is found in low levels in all our tissues, into ammonia, which is alkaline and neutralises the acid around the bacteria.

Non-ionising radiation is not as dangerous as ionising radiation.  Ionising radiation is the sort of radiation that we know does damage to cells like x-rays, gamma rays and beta rays which can cause damage to the DNA in your cells and cause cancer.  Ionising radiation is so called because it produces ions by interacting with the atoms in your cells.

Non-ionising radiation doesn't have enough energy to do this, so it doesn't damage your cells in the same way and is generally regarded as relatively safe.  An example of non-ionising radiation is, in fact, visible light.

Heart disease is when the arteries of the heart become blocked.  If you have a heart attack then the arteries have become so blocked that some of your heart muscle becomes starved of blood and can die.  At the moment we have no way to re-grow heart tissue, although muscle stem cells show promise.

There are ways to clear the blockage and open up the arteries again.  One way is to open the chest and perform a bypass operation, where a bit of vein from somewhere else in the body, usually the saphenous vein from the leg, is grafted on as a bridge to allow blood to flow around the blockage.  The newer way to do this, and much less invasive, is to thread a line up through the femoral artery in the leg and into the heart.  Using a scanner to see where you are, you put a wire inside the coronal artery and blow up a baloon which squashes the blockage out of the way.  You can then prop the artery open using a metal cage called a stent.  New stents are available which resease drugs to stop further blockage of the artery.

The Apophis asteroid is due to fly past the earth in 2029, and there is some speculation that it could come back and collide with the Earth later.  A company in Stevenage have announced that they would like to build a spacecraft to go and rendezvous with the asteroid to discover more about it.  By flying up to meet the asteroid we can find out far more that we can from Earth, and if we find out early enough about it's projected course we have time to try and knock it out of the way!  Should you build a shelter, probably not, but you've got 30 years in which to do it!

There are a number of factors here, one of which is the size of the animal.  Mice life, on average, about 6 weeks in the wild, though they can live up to 2 and a half years in ideal conditions.  A human, however, can live well in excess of 80 years.  There must, therefore, be a genetic element to this, as mice and men are separated by a small difference in genetics.  Other factors such as lifestyle decisions affect your lifespan, but longevity does seem to run in families.  If your family has large numbers of supercentenarians (people who are over 110) then you have a higher chance of living a long life.  There's increasing evidence that your cellular metabolism also affects how you age.

Tortoises can live much longer than you or I, but this is partly because they are cold blooded.  This means they have a slower metabolic rate and use less energy.