Related stories Physics, how we love you. You keep our feet on the ground, ontologically-ambiguous cats locked in boxes and photons in two places at the same time. It also enables the Peltier effect, what's that we hear you breathlessly ask? If a current is passed between two different materials a heat differential is created; one side hot the other cold. The principle is used in all sorts of situations including cooling processors and mini-fridges, though apparently they're not that efficient, just small and light. So why do we care? Well you can pick up a Peltier heat pump on eBay for under 5, we're interested in the 30W ones, these run at 5 to 12 volts so can be powered from a USB port. Also don't worry, the 'hot' side on these won't go over 40C under these power conditions, so the passive cooling we're going to use is perfectly suitable. Slap this between two old heatsinks and hey presto instant hot and cold fronts! We also picked up a couple of A3 sheets of 5mm foam board using this we can construct an insulated box, pack polystyrene around the base of the cooling heatsink and we've got an instant mini-fridge! For better cooling you could connect any suitable (sub 12v, sub 2a) spare DC adaptor to it, but if you do this will increase the hot side to over 60c. So double-checking the cooling and adding a small fan would be advisable. 2. Wire up the Peltier heat