Fill up a large sock (we use white crew socks) with white rice so it's stuffed good and full, and tie a knot. Throw it in the microwave for about 3 minutes, then into your sheets. It warms them up, and keeps giving off heat for hours. We've had visitors recently and everyone loved them! Perhaps they were mumbling what a cheap bastard I am for keeping a cool house, but I digress Can't remember where I heard of this, but it works.