Here's why:

* Diffusion: This is a passive process where substances move from an area of high concentration to an area of low concentration. This is how animals take in oxygen, which is more concentrated in the environment than inside their cells.

* Active Transport: This process requires energy to move substances against their concentration gradient, from an area of low concentration to an area of high concentration. This is important for taking in nutrients like glucose, which might be less concentrated in the environment than inside the cell.

Other processes like endocytosis (engulfing substances in membrane-bound vesicles) and osmosis (water movement across a semipermeable membrane) also play roles, but diffusion and active transport are the primary mechanisms for substance intake in most animals.