Testicles is a less offensive world for "bollocks". They are also referred to as "balls" or "nuts". They can get cancer. They produce a hormone known to cause male humans to make irrational decisions, as well as give them Bigfoot-esque body hair and an unpleasant odor.
They often cause male humans to buy trucks with four wheel drive even though they only use their vehicles to get back and forth to their places of employment. Sometimes the trucks buy testicles too.
They play a reluctant starring role in many videos shown on America's Funniest Home Videos and similar programs.
By way of proving that God (or FSM or IPU) has a sense of humor they turn out to be the single itchiest place on the human body, followed closely by that spot in the middle of the back that you can not reach. Further proof: It is perfectly acceptable to ask your significant other to scratch your back.