Simplify the following text for a Grade 13 native English-speaking person.
Original: Thermometers are another application of thermal expansion â '' most contain a liquid ( usually mercury or alcohol ) which is constrained to flow in only one direction ( along the tube ) due to changes in volume brought about by changes in temperature .
Simplified: Thermometers are an example of an application of thermal expansion â `` they contain a liquid which is constrained to flow in only one direction ( along the tube ) due to changes in volume brought about by changes in temperature .