What would make the temp gage go down when the car is still running and was at...

[Denese]

...between cold and hot? I just bought a car and when we were driving it took a long time to warm up. it finally got to the middle of cold and hot about 190 degrees and when i parked and left it running the temp gage went back down to cold. why would this happen. i started driving it again and it started to warm up. The heater works better the warmer the car gets too. This is after a hour of driving around town. plz enlighten me