In classical conditioning, when the conditioned response is extinguished, but reappears later, the reappearance of the response is called

Spontaneous recovery

Explanation:

A conditioned stimulus can be defined as the stimulus which can trigger the same response that can be created by a natural stimulus or unconditioned stimulus. It is a neutral stimulus that with a particular time and training evokes a response along with the naturally occurring stimulus.

The spontaneous recovery is a type of learning and conditioning process in which a specific response given by the organism reappears which was once extinguished. This recovery may happen when the conditioned stimulus is not exposed to the subject.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom