Circuit bending is about the non-thereoretical exploration of sound making circuits via shorting different points together. Take a toy (that is battery powered - important!) and let's get to work!
Finding the Clock
One of the most basic ways in which a cheap, musical or sound making toy can be circuit bent is by changing the clock in various ways. "The clock" refers to two points on the circuit board that are by default - connected via a resistor. However, this resistor can be removed, changed, replaced, modified.
To find the clock of a (battery operated) toy, first of all dismantle the casing so that you can reach the innards. You will need clear access to the circuit inside.
Slightly wet your fingers, and run them across the circuit whilst making making sound with the circuit. Eventually, you should find a resistor that - when touched on both terminals - will change the pitch and speed of the sound.
The resistor may be a through-hole resistor like this:
Or it may be a surface mount resistor like this:
Example: Finding the Clock of a Music Mat Toy