To take an ideal position for take-offs.
They hang the roof라고 하면 지붕을 걸어버리는것..
Small and weak legS. 다리 하나?
When they are in a dead stop.
They can get ready. Ready 동사 x

In the lecture, the professor describes why bats hang upside down. There are two reasons.
 The first reason is to hide from dangers. They hang on the roof of cave or inside tree during daytime so that their predators cannot see or reach them.
 The second reason is to take an ideal position for takes-off. Since they have small and weak legs and wings, they cannot speed up to take-off when they are in a dead stop. By hanging up-side-down, they can get ready to take off.
 These are two reasons why bats hang upside down.