The most common type of thermometer consists of a glass tube that has a small reservoir or vial at its lower end.. The reservoir contains a liquid. When you put the thermometer in hot water, the fluid dilates or expands. So, the liquid does not fit in the container and some begins to rise up the tube. The more the liquid heats up, more goes up the tube. Stop rising when it's hot as water.
If you put the thermometer in cold water, fluid decreases in volume. In the tank there is again space for the liquid and this is contracting in the container. The more the liquid cools, the more it contracts and there is more space in the tank. It stops contracting when it is at the same temperature as water. In some thermometers, the liquid is silver in color. This liquid is called mercury. The thermometer used to take people's temperatures is usually made of mercury..
In other thermometers, that liquid is red. This liquid is colored alcohol. The thermometers that are used to measure the temperature of the air outside are made of alcohol.
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