Inside each of your lungs there are tubes called bronchi. these branch into even smaller tubes much like the branches of a tree. what are at the ends of these tubes?

Small air sacs.

Explanation:

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.