What are surfaces and membranes ?

Surfaces are everywhere: the computer screen in front of you has a smooth surface; we walk on the surface of the earth; and people have even walked on the surface of the moon.

Surfaces can be flat, like a table top, or curved like the surface of a football, a balloon or a soap bubble. The surface of water can be either flat without ripples, or curved when it has ripples or waves on it.

We use the word membrane to mean a sheet-like object, an object with area but very little or no thickness. Good examples are sheets of paper or a piece of plastic food wrap. Just like surfaces, membranes can be flat or curved; rough or smooth.