Many situations require you to tie a closed loop to some other object--for instance, attaching a rubber band to a chain. Unlike a piece of string, a loop has no free end, so the usual knotting techniques are inappropriate. Here's a quick "knot" which is useful in most loop-related situations:

I don't want to attempt ASCII art, so I'll demonstrate with words. Take a piece of string tied into a loop, and lie it flat on a table. Lay a pencil across the loop from left to right so that it cuts the circle in two and extends slightly outside the loop on each side. Grab the bottom of the loop, lift it over the pencil towards the top of the loop, then put it under the top of the loop and pull it away from you along the table. The string will tighten around the pencil, and your knot is finished.

The same technique works for securing other objects which are topologically similar to loops, for example handbags, one-legged G-strings and donuts.