1998 Ford Explorer Thermometer says engine is burning up

I have a 1998 Ford Explorer sport. Yesterday I was siting in traffic and the car started making an extra noise like a fan came on. I noticed the thermometer was higher than usual, but it was also a hot day. So I just thought it was because it was hot out. However today I was driving and the thermometer was very high and the "check guage" light was on. So I got home and let the engine cool of with the hood open. When I looked in the radiator it said it was full. I started the car again and the temperature started going up very fast. People are saying its a thermostat problem or that I may have a leaky hose, but nothing is leaking.