What causes a person to sleepwalk?

The cause of sleep walking in children is usually unknown but may be related to fatigue, prior sleep loss, or anxiety.

Although the exact cause of sleepwalking is unknown, sleep experts believe sleep walking in adults can be associated with of triggered by mental disorders, stress, fever, reactions to drugs and alcohol, or medical conditions such as partial complex seizures and hormonal changes. In the elderly, sleep walking may be a symptom of an organic brain syndrome or REM behavior disorders.

The sleep walking activity may include simply sitting up and appearing to be awake while actually being asleep, getting up and walking around, or complex activities such as moving furniture, going to the bathroom, dressing and undressing, and similar activities. Some people even drive a car while actually asleep. The episode can be very brief (a few seconds or minutes) or can last for 30 minutes or longer.

It's called sumnambulism. It's a sleeping disorder where a person does an activity which is actually done normally during wakefulness while he/she is in a sleeping state. It's a bit dangerous though because it causes accidents. And its a misconception that sleepwalkers does the activities that they dream about because sleepwalking occurs before REM or the dream state. Usually the sleepwalker is unconcious of what he/she does unless he/she is awakened by someone.

Their feet.

http://www.guardian.co.uk/lifeandstyle/2002/sep/24/healthandwellbeing.health1
I didn't read this all the way but this is kind of funny in the beginning :)