When Baby Shane crawls towards the edge of the staircase, his mother screams, which startles Shane, and makes him cry. This happens several times, and now whenever Shane passes by the edge of the staircase, he feels afraid. In this example of classical conditioning, the conditioned stimulus is:

The conditioned stimulus is the staircase.

Explanation:

In classical conditioning, a conditioned stimulus (the staircase) is associated with an unrelated consequence called unconditioned stimulus (the screaming). This produces a reaction known as conditioned response (fear).

It's important to realise that any stimulus can become conditioned. In the example, if Shane's mother had screamed at the sight of a dog, the baby would probably be afraid of dogs.