THIN GLASS IT MORE LIKELY TO CRACK THAN THICK GLASS WHEN YOU POUR HOT WATER INTO IT
OR....THIN GLASS IS MORE LIKELY TO CRACK THAN THICK GLASS IF IT IS BENT WRONG!!
Glass is BRITTLE which means that it cannot distort very much before it breaks.
Imagine you have a glass rod held infront of you and you try to bend it so it goes up in the middle and down at the edges. What happens is that you are STRETCHING the top edge and SQUASHING (compressing) the bottom edge and the glass cannot "take" the huge distorsion across its edges so it snaps.
If your piece of glass is very thin however, the DIFFERENCE in length between the top and the bottom edges is much less, so it can bend through a bigger angle before it snaps.... ie: optical fibres are bendy though they are still glass.

When you pour hot water into a glass, it heats the inside of the glass and therefore makes it expand with the heat. BUT.....glass is not a good conductor of heat, so if the glass is thick, the inside will start to expand while the outside (still cold) will not..therefore it cracks. If the glass is made of thin glass, then it is more likely that the heat will be able to conduct through to the outside and make the outside expand as well as the inside so it may not crack.

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