Tempered glass is manufactured in a way to make it stronger. It is heated to extreme temperatures and then quickly cooled, which changes the molecular structure of the glass.
In The Dishwasher
Because of its strength and resistance to breakage, tempered glass dishes and tempered glass drinking glasses are dishwasher-safe. They can withstand the extreme temperatures of the dishwasher as well as the movement of the dishwasher's cleaning process and rattling from the racks moving in and out.
Tempered glass is found in items that are meant to withstand extreme temperatures, such as casserole dishes, oven doors and coffee pots. There are tempered drinking glasses available, but be careful. Some drinking glasses are only tempered around the rim, not the whole drinking glass. Fully-tempered glasses and other tempered glass dishware are dishwasher-safe.
One property of tempered glass that makes it especially safe for the dishwasher is how it breaks. Although it is stronger than standard glass, it can still break. When it does, it breaks into small, round pieces instead of dangerous and sharp shards. This makes retrieval from the dishwasher safer and easier if the tempered glass does happen to break.