The Chemical Store Fire

A chemical store burned down, and the firefighter just stood there, but at the end, the store fire just went out by itself. But the store owner still got angry.

Store owner: Why didn't you take out the fire?

Firefighter: Yeah, but it went out by itself.

Store owner: But still, why?

Firefighter: Your chemical store sells H20.

Store owner: Oh, I get it now!