Why is it that when you pour cola in a glass with ice, there's more foam than when you pour it in a glass with no ice?

Can two black holes merge together if they get close enough to each other?

How is it that visible light and radio waves are harmless, but microwaves and gamma rays have adverse effects on life? There seems to be no linear path from the least harmful to the most harmful in the spectrum. The harmless waves seem to be interspersed with the harmful ones as you climb up the spectrum. Is there an explanation for this?

Why does water bubble when it boils?

At this time of the year you often get cold hands. A quick but painful way of warming them up is by sticking them into warm water. If pain is a sign of damage, is this actually damaging your hands and what's happening when your hands are coming back to life?