A watch glass is a circular piece of glass used in scientific laboratories as a surface to evaporate liquids, hold solids during weighing, and cover beakers, flasks, and other vessels. It is typically made of clear or frosted glass and ranges in size from a few centimeters to several inches in diameter.

The shape of a watch glass is slightly concave, which allows it to hold small amounts of liquid without spilling. Watch glasses can be reused after being cleaned and sterilized, making them a cost-effective and practical tool in laboratory settings.