When making bread, the recommendation is to place the dough in a warm spot to rise for about one hour. Since we hardly heat the house in winter (the thermostat is set at 12°C; the living room is a bit warmer still) this proved a bit of a challenge.
Insight came when feeling around for sources of heat in the living room. The sun shining on the couch, a warm lap (the favorite spot for the cat in winter), and... the internet router.
The router gives off a steady flow of (low intensity) heat. Placing the metal bowl on top of the router allows for a good conductance of the heat to the dough. It works quite well. It is also a good example of the attitudinal principle Multiple elements x multiple functions.