Find the hole: Inflate the tube listening for a hissing sound and follow it to find the hole. If you can't hear a hiss, hold the tube next to your face (or lips; they're especially sensitive) as you search for the hole, and you should feel the escaping air. If you can't find the hole, try adding more air. Still no luck? It's probably a really slow leak. To find it, submerge the tube in water and look for bubbles (photo). Can't inflate the tube? There's probably a hole in it so large that the air leaks out immediately. Look for it or have a friend listen and look while you pump.

Tip: If the tube has a tear 1/2-inch long or longer, has several holes in it or has a hole next to the valve, it'll be difficult to impossible to patch. Time to buy a new tube. Use the old one to make a water-balloon slingshot.