The Temperature Sensor works by monitoring how cold or how hot a specific location in your house is. Its thermostat companion can then make sure that room stays at the temperature you specified -- like say, if you want your bedroom colder than the baby's room or your living room a bit warmer than everywhere else -- for max comfort. It sounds especially useful if you have a big house, which is most likely why Nest is offering a 3-pack bundle price for $99. You can still get just one sensor for $39, though, if you'd still like to make sure you don't get too hot or too cold in your apartment.