This is because the ice is sharp and is something called nucleation. Coke has lots of dissolved carbon dioxide in it, which is what makes it fizzy. When you put it in a glass, you'll notice that the bubbles all come from little spots in one place on the surface of the glass and they stream up in a stream. The reason that they do that is because there'll be a little area on the glass or a tiny imperfection that makes the glass slightly rough and it's easier for a bubble to form there. The carbon dioxide dissolved in the drink starts to come out of solution and come out as bubbles of gas at that point. If you put ice in the glass, which is very very rough and has lots of sharp edges, it creates an even bigger surface area with lots of nucleation sites and you get lots of gas coming out. This is why it becomes frothy.

Yes, can black holes can merge. When two black holes get quite close, it actually becomes inevitable that they'll end up spiralling in towards each other and merge together. This causes a massive energetic event. What you get at the end of it is just another black hole, but bigger. But what it can also do is form ripples in space. If you know anything about relativity, then you know that gravity is caused by bending of space. When you merge two black holes you get ripples, but so far they're only a theoretical idea and scientists are trying to look for them. If we could spot them, then that would really prove relativity.

There are two different mechanisms going on here. The first is that as you get higher frequency waves, such as gamma rays and UV waves and X-rays, what you actually get are more energetic waves. They can cause ionisation, which is incredibly bad, and DNA damage and in that case problems for life. The energy of those waves is powerful enough to rip molecules apart. Microwaves are something slightly different. Microwaves are bad because water specifically absorbs microwaves and converts the microwave energy into heat. That's how your microwaves work at home. The microwaves are absorbed by the water in the food, which creates heat, so if you're irradiated by high-intensity microwaves, you're actually cooking yourself. However, microwaves are non-ionising so it doesn't damage your DNA and it can't trigger cancer like ultraviolet radiation could or gamma rays or x-rays.

You have an element in your kettle and that gets very hot. Next to that element is water that turns to water vapour, which is 1000 times bigger than water because it's a gas. This bubbles up through the liquid.

I guess it's partly because the cold itself produces quite profound chemical changes in and around the nerves that report pain messages, and that's why anybody who's had cold hands for a long period of time will describe how intensely painful it is. So I suspect that it's because the nervous system is already in a sensitised state when one adds another signal to those nerves. What nerves do that aren't behaving properly is they misinterpret sensory information, and so my guess is that partly what's happening is that you've got nerves that have been upset by the cold and when you apply a warm stimulus, it might be interpreted as being painful. This doesn't necessarily mean that it's damaging. It may also be something to do with mixed signals going to the brain, so you have messages coming that they are very cold, but then you get a mixed signal about another sensation coming in. Often these signals can be interpreted as pain. I don't know if that's the right answer but that's what I would guess.