Before going outside in the morning, many of us check a window thermometer(温度计)for the temperature. This helps us decide what to wear.        We want our food to be a certain coldness in the refrigerator. We want it a certain hotness in the oven. If we don’t feel well, we use a thermometer to see if we have a fever. We keep our rooms a certain warmth in the winter and a certain coolness in the summer.
Not all the thermometers use the same system to measure temperature. We use a system called the Fahrenheit scale. But most other countries use the Centigrade scale.
Both systems use the freezing and boiling points of water as their guide.       
The most common kind of thermometer is made with mercury(水银)inside a clear glass tube. As mercury (or any other liquid) becomes hot, it expands. As it gets colder, it contracts. That is why on hot days the mercury line is high in the glass tube.      
First. Take a clear glass juice bottle that has a cap; fill the bottle with coloured water. Tap a hole in the center of the cap using a hammer and thick nail. Put the cap on the jar. Then stick a plastic straw(吸管) through the nail hole.
      
Finally. Place a white card on the outside of the bottle and behind the straw. Now you can see the water level easily.
      
As the temperature goes down, the water will contract, and the level in the straw will come down. Perhaps you will want to keep a record of the water level in the straw each morning for a week.

A．Thermometers measure temperature, by using materials that change in the same way when they are heated or cooled.

B．We use and depend on thermometers to measure the temperature of many other things in our daily lives.

C．Take wax (you may use an old candle if you have one) and melt some of it right where the straw is struck into the cap to seal (把……粘住) them together.

D．The water will rise in the straw. As the temperature of the air goes up, the water will expand and rise even higher.

E．They label these in different ways. On the Fahrenheit scale water freezes at 32 degrees and boils at 212 degrees. On the Celsius scale water freezes at 0 degrees and boils at 100 degrees.
F．Now that you know this rule you can make a thermometer of your own that will work.
G．People use thermometers which are made by themselves when travelling around the world.