If you do—crack! You may break it. If the glass is thick it is almost sure to crack. The thinner the glass the smaller the risk. That's odd, isn't it? Why is this? Heat expands. When a thing expands it needs more room. Glass expands readily. If it is thin the heat goes through quickly, expanding all parts alike. But when the glass is thick, the inside particles expand before the outside becomes heated. So the cold outside layer has to crack to give the warm inside layer room to expand. The same thing happens, sometimes, in pouring hot coffee into a cold cup. If you put a silver spoon in your glass or cup it will be less liable to be cracked. The metal attracts the first heat, and allows the glass or china time to heat more slowly and evenly. And a glass or china cup is apt to be cracked if it is very hot and you put ice water in it. Cold contracts or shrinks the glass. So the inside shrinks while the outside is still stretched.