What is the tool to measure how flat a surface is?

Question

What is that little liquid filled thingy used to see if the surface is flat? I've usually seen these placed on top of the wood surface in construction such as when someone's building a patio , to see if it is even. Also where will i be able to buy one?

Answer 1

A level. You can purchase at any hardware store, walmart, etc.

Answer 2

It is a level, and they come in a zillion sizes.  Any hardware store or Home Depot, Lowe's, etc.

Answer 3

A level, Home Depot or any tool and hardware store.