A meniscus is the curved surface at the top of a column of liquid. In a science class, this liquid is usually water or some sort of aqueous solution, and the column is usually a graduated cylinder or a pipet. As you may have noticed, when water is in such a thin glass tube, it does not have a flat surface at the top. Instead, the top is curved inward, making it a little difficult to decide exactly where to read the volume. As pictured below, the volume should be read from the bottom of the meniscus.
Why doesn't water lie flat? Well, it has to do with the nature of the water molecules and glass molecules themselves. Water is made up of polar molecules, which have positively and negatively charged ends. Since opposites attract, the positive sides attract the negative sides, and all of the molecules stick to one another. This is why water droplets can form. Glass molecules also happen to be polar. Again, since polar molecules like to stick together, the water in a glass tube will actually tend to stick to the sides of the tube! You can see this at the top of the graduated cylinder, where the water will slightly creep up the sides and form a curve, which is the meniscus.
(republished on 08/01/06)