Here's why:

* Size: Cells have different sizes depending on their function. For example, nerve cells (neurons) can be very long and thin, while red blood cells are small and round.

* Shape: The shape of a cell is also determined by its function. Muscle cells are long and thin to allow for contraction, while skin cells are flat and tightly packed to provide protection.

Let me know if you'd like more details on specific cell types and their variations!