Here's why:
* Endo means "within" or "inner."
* Exo means "outside" or "external."
This prefix pairing is commonly used in scientific and medical terminology, such as:
* Endocrine (hormones produced within the body) vs. Exocrine (hormones released outside the body)
* Endocytosis (bringing substances into a cell) vs. Exocytosis (releasing substances from a cell)