An accurate temperature reading is important when dealing with draft beer so knowing how to calibrate a thermometer is a good skill to have.

What you will need:

Dial Type Thermometer
 A glass about as tall as the thermometer
Water
Ice Cubes
Pliers

What you do:

Fill a glass with ice and then top it off with water. Add some more ice so the glass is full and insert the thermometer into the ice/water mixture. Let it sit for 15 minutes or so and note the temperature. If it reads 32 degrees then you are all set, if not you will need to adjust the settings. Do this by grabbing the nut on the back of the thermometer with pliers and turning the face to adjust the needle to 32. For the most accuracy do this step fast (or with another pair of pliers) as your body heat will cause the thermometers temperature to rise. Recheck calibration by placing in glass again and verifying the 32 degree reading. Repeat until correct.