Buddhism compares the untrained mind to a chattering monkey that jumps from one branch to another. The monkey is never still and constantly moving. This is not a flattering image, but if you watch your thoughts darting from one random idea to another, you may discover that it is quite accurate. You can train yourself to be aware of the stream of thoughts passing through your head, and to quiet the chattering monkey.
— Source: 101 Essential Tips: Meditation (DK Publishing, 2004)