Jogging on a beach where the sand is packed together, near where ocean waves meet the sand, you notice that your footprints quickly fill with water after each step. waves are not filling them up. instead, there is another reason. where is the water coming from?

The sand particles have many interconnected pores between them. As a result of which they have some porosity. When water is added to the sand, then the water gets percolate into those tiny voids between the sand grains. Once these spaces are fully filled up with water then it becomes saturated. This means that the sand particles have no more capacity to hold water in its spaces.

This effect can be seen very commonly in the beaches, where the sands are fully saturated, and as someone steps over it, the water starts to come out through this pore spaces. This water that comes out of these footprints is the water that is stored in those tiny pore spaces in a fully saturated condition.