Put a plastic or glass tube in a glass full of water. The level of water in the glass tube rises higher than the level of water in the glass. Why?
To study its cause thoroughly, take three or four glasses or plastic tubes of different diameters and put them in a glass of water. You will be surprised to see that in all the tubes, the level of water has risen to different heights but the water in the glass is still quite steady and still. The water level in the tube of the least diameter is the highest whereas it is lowest in the tube of the maximum diameter. Why does it happen?
Take a test tube or a long, narrow and cylindrical bottle and fill half of it with water. Now observe the level carefully. It is highest at the sides and gets gradually concave in the middle! This curved level of water is called meniscus and the reason of its such formation is the force of attraction exerted by the walls of the tube or the bottle on some portion of water due to adhesion. Normally, you can reckon this adhesion to be a force which raises the level of water on the sides of a tube. Actually, this action of raising the water level higher is called the capillary action. This capillary effect is strongest on the tube with the least diameter because in this condition, a large portion of the Owing to the adhesive force, it remains in the fold of the attraction working on it.
Now see what happens if you apply grease inside the tube before filling it with water. This time the level of water instead of curving downwards would seem to be curving upwards. Why? It is so because the molecules of water do not get attracted as much by grease as by the plastic or glass. In this case, the mutual attraction of water molecules is more than the attraction caused by the grease, which is known as cohesion and not adhesion. water remains in direct contact with the sides of the tube.