double wall glass is made from hand blown borosilicate glass, lighter and thinner than other mugs, but more durable than normal glass and more sturdy than plastic or melamine.

There is a small round hole design on the bottom of the glass coffee cup, in order to prevent glass mugs burst when using, allows the glass to breathe in the process. It is sealed, the water would not go in.

100% Lead-Free Borosilicate Glass
Can withstand the temperature from -30 to 150°C
Lead-free, No toxic, No Plastic
Easy to clean and durable