This extra jostling pushes the atoms farther apart, causing the whole substance to slightly increase in size. If the substance cools off, its atoms stop pushing against each other so vigorously. This makes the substance shrink again. This connection between temperature and volume is what makes most thermometers work. The most common type of thermometer is an airtight glass tube with a reservoir bulb at one end. The reservoir is filled with a colored liquid. As the temperature increases, the liquid expands, forcing itself partway up the narrow tube. The hotter it gets, the higher the liquid climbs. Numbers printed along the tube translate the liquid’s volume into temperature, and you learn whether to wear a sweater or a short sleeved shirt.