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).
Step 2: Make It.
Make a small hole in the middle of the cannister's lid, put the tube through it and seal around the tube. When you put the lid back on, the tube should almost reach the bottom of the cannister, but not quite.
There are three ways to make the hole:
Drill really carefully: use a small sharp blade to drill a perfectly circular hole, stopping frequently to try the tube through it. When you can just get the tube through, stop drilling.
Drill quickly: use whatever tool you have handy - pocketknife, scissors, particle-beam accelerator - and then seal any gaps between the hole and the tube with blu-tac, modelling clay or hot-glue.
Melt it: heat a narrow nail in a flame (hold the nail with pliers or you'll burn yourself), and poke a hole in the middle of the lid. Quickly push the tube through the hole and, as the plastic cools, it will set and make an air-tight seal around the tube.
Did you push the tube too far through before you sealed it, and now you can't close the lid? Never mind, just snip the bottom of the tube off, no big deal.
Step 3: Fill It.
Fill the cannister about a third or half-full with coloured water and push the lid on.
Be careful, because the water can easily squirt out the top of the tube.
You should end up with the coloured water part-way up the tube with plenty of free space above it.
Step 4: Try It.
That, basically, is that. The thermometer is finished and works.
Hold the cannister in your fist - the warmth of your hand will make the air in the cannister expand and push the water up the tube. Dip it in your coffee, it will go up further. Stand it in your fridge and the air will cool and contract, pulling the water back down.
Obviously you cannot use the thermometer to check your freezer compartment or your sauna (think about it...), but you are now the proud possessor of a working model of an early Fahrenheit thermometer.
Step 5: Calibrate It.
Thermometers aren't much use without a scale. You can have one as well.
Tape the card against the tube like a flag, then hold the cannister in your fist. When the water stops moving, mark the point and label it "hand heat". Try a range of other places - a cup tea just cool enough to drink, slip it under the quilt with your loved one in the morning. One of Fahrenheit's original standard points was "human armpit".
When you have enough points on your scale, add some numbers. It doesn't really matter what they are, either, since Fahrenheit's scale was quite arbitrary - he just used numbers that "felt" right, which is why water freezes at 32F and boils at 212F.
Give your scale a name, then take delight in confusing your friends when the conversation inevitably drifts round to the weather;
"Boy, it's hot today!"
"Yeh, it was nearly forty Desmonds when I checked this morning."