I sleep in a Queen-sized bed complete with a warm and snuggly electric blanket.

When I go to bed, I set the blanket to "4" and all is right with the world. In the morning, the blanket is still working away keeping me all toasty warm. I never, ever remember to turn it off, but when I come back a couple hours later, it has turned itself off.

How does it know I am no longer in the bed?

tanstaafl.