Why Does a Glass Crack When Hot Water Is Poured into It?
An ordinary drinking glass may crack if you pour hot water into it. Glass expands, or swells, when it is made hot.
When you pour boiling water into a cold glass, the inner surface of the glass grows hot and expands very quickly, while the outer surface remains cool. The uneven strain is so strong and sudden that the brittle glass is cracked right through.
If, however, the glass be previously washed with a little warm water, so that the whole is gradually heated, and, therefore, gradually expanded, then the hot water may be poured in without danger ; because, although the bottom will expand as before, yet the sides also enlarge, and the whole vessel undergoes a similar change of heat.
Another option is to put a spoon in a glass before pouring in hot water which helps to prevent a glass from cracking. The spoon sprays the hot water against the sides of the glass, causing the sides and bottom to heat more evenly.