Digestion – in the stomach.
The stomach is full of hydrochloric acid, a Highly Corrosive Liquid (HCl), and is an ideal environment for the protease enzyme (pepsin) to work on the proteins.
The stomach is like a swimming pool of fluid with hydrochloric acid in it. The protesting teenagers (proteins) won’t get into the fluid because it burns them. All the protesting annoys the life guards and disturbs them drinking pepsi all day (pepsin). So the life guards throw them out.
Although, again, the stomach muscles help to physically break up all the food, it is the pepsin enzyme in the hydrochloric acid which chemically breaks down the proteins.