Question

drag the missing word into place
air is made of molecules. when air is cool,
these molecules are tight and stay closely bound.
and when the air is warm, the molecules are less

Explanation:

When air is cool, molecules are closely bound (more dense). When warm, molecules have more energy and spread out, so they are less dense. "Dense" relates to how closely - packed molecules are. "Windy" refers to air movement and "airy" is more about a light, open quality (not related to molecular density).

Answer:

dense