To purify from defilement (caused by eating forbidden food).
In order to purify a person from a defilement (and to make him ritually clean again), the imam or any person who knows the correct formula gets soil out of a hole that is made forearm deep, mixes it with water and gives it to drink. A bath in connection with the correct formula can accomplish the same thing.
Please purify my child; he ate something defiling.
He purified people there at the river.
You have eaten something forbidden, you are not allowed inside the mosque if you have not been purified.
The chicken knows how to purify itself.