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. _____76____. 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._____77_____ . 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(收缩). That is why on hot days the mercury line is high in the glass tube._____78______. 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. ______79______. Finally. Place a white card on the outside of the bottle and behind the straw. Now you can see the water lever easily. ______80____. As the temperature goes down, the water will contract, and the lever in the straw will come down. Perhaps you will want to keep a record of the water lever in the straw each morning for a week.