Try this: Make your own thermometer

You will need

• A tomato sauce or mayonnaise squeeze bottle. You can use a plastic container or bottle with a water-tight lid but you will need to make a hole in the lid.
• Blutak or plasticine
• A clear, thin drinking straw
• Food colouring
• Waterproof marking pen
• Water
• A wooden skewer (optional)
• Eye-dropper (optional)

What to do

1. Half fill the bottle with water and add a few drops of food colouring. You may need to add more water depending on the size of your bottle and length of your straw.
2. Place the bottom of a straw in the bottle so it touches the water. The top of the straw should be sitting well above the mouth of the bottle.
3. Holding the straw in place, tightly seal around the straw and the top of the bottle with plasticine. Be careful not to crush the straw.
4. Blow a little air into the bottle which will cause an amount of the coloured water to rise up into the tube above the stopper. Be careful when you blow into the tube. If you blow too much air into the bottle a jet of water will squirt back at you.
5. If the water level in the straw drops, it means air is escaping through the seal. You need to make sure you have no leaks in your seal.
6. When there is water in the straw you may need to remove any air bubbles inside the straw by moving a skewer up and down in the straw. 
7. If the water in the straw doesn't rise high enough use an eye-dropper and fill the straw with water to a level of about 5 cm above the top of the bottle.
8. Mark the level of the water in the straw with a pen.
9. You have now calibrated your thermometer to room temperature.
10. Cup you hands around the bottle or place it near something warm in the room. Be careful not to place plastic too close to a heat source or it will melt. What do you notice about the water level in the straw?
11. Place the bottle in the fridge and after about ten minutes take it out and look at the water level in the straw.

What's happening?

The thermometer works on the principle that when something warms up it expands and when it cools down it contracts.

If you raise the temperature of a gas, the particles that make up the gas absorb heat energy and begin to move faster. This causes the gas to expand. When the air inside the bottle expands, the pressure inside the bottle increases, pushing down on the liquid inside the bottle and pushing more liquid up the straw. When you cool the air again, it loses energy and decreases the pressure. The coloured water will then be pushed back down the straw by the air outside.

A simple bulb thermometer works on this principle.

Standard thermometers use alcohol. Liquid alcohol contracts upon cooling and expands upon heating. Alcohol has a lower freezing point than water, so it will measure temperature below freezing. Ask a adult, they might notice when certain alcohols are stored in the freezer at home they remain a liquid. A bulb thermometer uses a very small amount of liquid so that it changes temperature quite easily and the tube is extremely small, so slight changes are easily noticed.

Applications

A method for measuring temperature has only been around for about 4 centuries. The 'thermoscope' was invented by Italian mathematician Galileo Galilei in the late 15th Century. By the 1630s, scientists developed a thermometer. The basic design - a fluid-filled hollow glass bulb attached to a stem with a thread-like bore - is still used today.

Thermometers are calibrated by the measuring the distance between the freezing and boiling points of water. The Celsius scale uses 100 degrees between these two points. A Celsius degree (C) is 1.8 times bigger than a Fahrenheit degree (F). Water freezes into ice at 0ºC which corresponds to 32ºF, while liquid water boils into steam at 100ºC which corresponds to 212ºF.

Anders Celsius was a Swedish astronomer who suggested that temperature should be measured on what we now call the Celsius scale. In Australia we use the Celsius scale.

More information:

Great Moments in Science - Coldest Place in the Universe: Part One and Part Two