Level a picture or mirror by holding a half-full glass of water against the top edge. The water surface will be level even if the glass itself is tilted.
For long-distance levelling, make yourself a water leveller. Use a length of transparent plastic tubing attached at one end to the neck of an upside-down plastic bottle, with its bottom cut off, as shown. Fix the bottle at the right height to a ladder. Pour water into the bottle until it reaches the desired height at both ends; mark the spot on the tube. No matter how far apart the bottle and the end of the tube are, or how irregular the ground in between, the two will be at the same height when the water is aligned with the mark on the tube.
