A water molecule is like a weak magnet that momentarily attaches to other water molecules. This process of continuous momentary attractions is what makes cohesion possible. A water molecule is made of one big oxygen atom that holds onto two smaller hydrogen atoms -- shaped like an L. The oxygen atom gives the corner the L a slightly negative electrical charge, while the hydrogen atoms at the tips have slightly positive charges. These charges give each water molecule a mini-magnet-like quality.
Cohesion and Adhesion
Cohesion is defined as the attraction of water molecules to other water molecules. This is in contrast to adhesion, which is the attraction of water molecules to a solid surface. Cohesion is done by water molecules that are in the middle of a drop or body of water. Adhesion is done by water molecules that are at the outer edges of a volume of water. In trees, cohesion and adhesion work together to move water that is absorbed in the roots all the way up to the leaves. Adhesion and cohesion allows water to climb up the narrow water tunnels that are inside of plants.
Water has bent shape, like a boomerang. The oxygen atom sits between two hydrogen atoms and forms the corner of the boomerang. Cohesion is possible because water has what is called a polar nature, meaning it has a north pole and a south pole. As previously described, these poles give water a magnet-like property. But why does water behave like a magnet? It is because of a chemical principle called electronegativity. Electronegativity is the ability of an atom to attract electrons towards it. Atoms are made of particles called protons, neutrons and electrons. Protons and neutrons cluster at the center of an atom and give the atom its weight. Electrons barely weigh anything and float around in a cloud around the protons and neutrons. Protons have positive electrical charges while electrons have negative electrical charges. The attraction between them is what keeps the electrons from floating away. Water has poles because the oxygen atom is more electronegative than the hydrogen atoms. Oxygen is bigger and has more positive protons, so it pulls the shared electrons closer to it.
Electronegativity is what allows a water molecule to have north and south poles, but hydrogen bonding is what makes cohesion possible. Hydrogen bonding describes the interactions between two or more water molecules. The rule of electrostatic attraction is that opposite charges attract each other. Since a cup of water contains many water molecules floating inside of it, the slightly negative oxygen atoms of water are constantly bumping into and momentarily attaching to the slightly positive hydrogen atoms of other water molecules. When two oxygen atoms run into each other they move away because of their similar slightly negative charge. The same goes for two hydrogen atoms from different molecules that bump into each other.
Ever wonder why a drop of water on a table looks like sphere? Water can do that because of cohesion at the surface of the droplet. The water molecules are constantly hydrogen bonding with each other within the droplet. However, the molecules in the middle of the droplet are busier than the molecules at the surface. This is because the molecules in the middle are being pulled from all sides, while the molecules at the surface are exposed to air on one side and only need to deal with other water molecules on their other side. Each water molecule has a certain amount of strength to pull on its neighbors. Since the molecules at the surface have less neighbors to pull on, their strength is divided among fewer neighbors. This means they can pull harder on their fewer neighbors, resulting in an extra tight packing of water molecules at the surface of the droplet.