We usually use shower gel by rubbing it on our palms and then applying it to each body part in turn in order to clean oily substances that clung to it since our last shower. This procedure does not cause the gel to foam (if it is applied to a completely hairless place, such as a clean-shaven head or back, it will produce some small amount of foam (also depending on how much you rub it); if it is applied to a place with relatively small clusters of hairs, such as arms or legs, it might foam up a bit more, and so on).
However, when applying shower gel or shampoo to a hairy place, such as the head, the groin, etc. it usually foams up significantly and rather quickly. A clear shower gel or shampoo will become a white foam in a short time.
Why is that? Is it the air trapped among the hairs?
(Please do not confuse this question with this one, they are different enough).