Why is it difficult to walk on sand?
3rd Law: action/reactionWhen you walk, the reaction force from the substrate (floor, sand, etc.) is what pushes you forward. On sand, the reaction force is divided over many, many small particles, and is scattered into a myriad of directions. This scattering does not leave a lot of reaction force pointed toward your foot, so a lot of your effort is scattered -- that is, lost.