Context : Hippos are generally not dangerous. But if a male elderly hippo is turned out of its herd, it becomes irritating. Sometimes we may pass through the herd of hippos when they are sleeping, in such a condition they may attack upon us.
Explanation : It cannot be said exactly how many hippos are there in a herd. They are always hidden under the water and come up after every few minutes when they want to breathe. So by counting their heads only, their number can be told. If the heads are seen coming up for a long time, it means that there are large number of animals in that herd. Then they can be counted by their heads.
