This water level is really useful for example when you need to paint a room of two different color horizontally, or you need to find the same level of two things far apart from each over, like two shelfs, or two paintings.
Step 1: WHAT YOU NEED
Step 2: CUT THE BOTTLES
Step 3: CUT THE INNERTUBE
Step 4: ASSEMBLE IT
Step 5: FILL IT WITH WATER
Step 6: DONE
Is easier to use in two people (or otherwise you need to make an hole in one of the bottle and hang it up on a neil on the wall).
Hold them both FIRM on the wall for few second and mark the water level with a pencil.
The water level is always constant (communicating vessels principle).