A sheet is a large, flat piece of fabric used to cover a bed. Most beds have two sheets: a bottom sheet that covers the mattress and a top sheet that goes between you and your blanket. Sheets are usually made of cotton or similar material that feels smooth and comfortable against your skin. When you “change the sheets,” you're putting fresh, clean ones on the bed.

The word also refers to any large, flat, thin piece of material. A sheet of paper is what you write on in school. A sheet of plywood is a flat board used in construction. A baker might roll dough flat on a cookie sheet. When rain falls so hard and fast that it looks like a solid wall of water, people say it's coming down in sheets.

In sailing, sheets are the ropes used to control the sails (surprisingly, they're not the sails themselves). When a sailing ship's sheets come loose and flap wildly in the wind, the ship becomes hard to control. This is where we get the expression three sheets to the wind, which describes someone stumbling around unsteadily, like a ship that can't keep its sails under control.