This thing always happen to me before. A cracking glass with water. To make it clear, its a cracking glass with a recently boiled water. A cracking sound was heard just before the water pouring was finished. A non-straight line was then seen on glass side. In some instances, the glass broke completely and the hot water spilled.

Mother thought me great solution for this. Place a metal spoon in glass before pouring hot water. When pouring to several glasses, each one should have its own metal spoon. This works for me every time.

Why glass breaks when poured with very hot water? Glass is brittle and it cannot resist a sudden rise in temperature. It cracks a result. The same is true with the sudden temperature drop.

Metal spoon is a good heat conductor. It absorbs heat from just boiled liquid, lowers its temperature and prevents cracking. Using the same spoon for consecutive glasses is not a good idea. It  already absorbed the heat of the first container and will not be effective in the next.

There are other tricks. Let the hot water stand for five to ten minutes before pouring. This will lower the water temperature. Or use a more resistant medium, a  ceramic coffee mug.