If you're putting up a shelf but don't have a carpenter's level, you can always "eye-ball it" -- just don't plan on putting anything on the shelf that rolls. We've got a better option for you, though, which will save you a trip to the hardware store and make sure your new shelf is a safe place to display your cherished collection of antique marbles.
Substitute a tall, straight-sided jar (like the kind green olives often come in) with a tight-fitting lid for the level. Fill the jar 3/4 full of water. Lay it on its side on the surface you're testing, but keep a hand on it so it doesn't roll off and shatter on the ground. When the water is level, the surface is level.
Adapted from "101 Old-Time Country Household Hints," © 2008 Publications International, Ltd.