How did the hipster burn his tongue?

How did the hipster burn his tongue?
He drank his coffee before it was cool.

This joke is a pun. It is based on the word cool. Hipsters are people that follow the latest fashion trends, especially those that are viewed as being outside of the norm, however it is often said about or by hipsters that they "liked things before they were cool".

Cool can mean both popular, and having a low temperature, so for the purpose of this joke, the coffee was both not yet popular, and still too hot to drink.