How did some mammals survive the KT extinction?

From Flora Ames

Mammals were generally pretty small when dinosaurs ruled the Earth; they lived in water or underground and they were adapted to eat a variety of foods. When the asteroid struck, the Earth heated rapidly, plant life died away and large animals were left hot and exposed with nothing to eat. Mammals were sheltered; they could regulate their body temperature; and they could eat insects, helping them to thrive in these new, challenging conditions.

Answered by Laura Mears for Brain Dump in How It Works issue 104

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