Can a turtle tuck its head all the way inside its shell?

Broad-shelled river turtle, Chelodina expansa, Cedar Creek, Brisbane, Queensland, Australia .

It's a long-held idea that turtles can tuck their heads into their shells when threatened. But is it true? And is this protective trick why turtles the world over have shells today?

The answer is that some types of turtles can, and others can't, experts told Live Science. And even though shells can be protective for some of these reptiles, fossil evidence suggests that shells evolved for entirely different reasons.