About 300 years ago, Daniel Fahrenheit made and calibrated the first reliable thermometers. Some people* still use the scale he developed; the Fahrenheit Scale.
*like Americans ;-)
Step 1: Materials and tools.
You need a small air-tight container (a 35mm film cannister is perfect), a thin translucent or transparent tube (such as an empty biro ink-tube), food colouring and something to make and seal a hole.
If you are going to add a scale, you will also need a piece of white card, clear sticky-tape and a narrow marker (an OHP pen is good).