The full text of what is said and shown.

A flamingo sleeps for hours on one leg

Never tiring

That leg isn't working

Its knee locks on its own — a mechanical catch, zero muscle

It can doze off standing up

So much so that a lifeless flamingo, well placed, still stands

Why one leg and not two?

The colder it gets, the more they tuck it up

The cold gives the reason

Biomechanics, the permission