Why lizards are not active on cold day?

Lizards and other reptiles are cold-blooded animals, meaning that unlike mammals and us, they cannot make their own heat by will to keep warm. Instead, their body heat goes with the temperature in the air, so if it's warm, the lizard's blood will be flowing and it will be active. If it is freezing cold, however, the lizard's blood will be so cold it will need to stay unactive to prevent its blood from being iceified.