Why do you use a coverslip when making a wet mount?

Coverslips are used in wet mounts to hold the specimen and its surrounding liquid in place and prevent it from bunching up in a droplet or sliding off the slide. Coverslips also prevent the lens from touching the sample.

One main goal when using a coverslip is to ensure an appropriate amount of liquid or sample on the slide. If there is too much liquid, the coverslip does not stay in place. Excess water may be removed by placing an absorbent material near the edge of the slip to draw water out. Lack of water under the coverslip can be remedied by placing a drop of water on the outside of the slip near its edge.