Dave has a pet fish named Guppy. Dave habitually turns on the fish tank's light every morning prior to feeding Guppy. After a while, Dave notices that Guppy swims right to the top of the tank as soon as he turns on the light. In this example, the is the CONDITIONED STIMULUS.

Turning on Lights

Explanation:

Stimulus is anything which leads to response. Unconditional stimulus are natural or automatic in human beings or organisms. Conditional stimulus are a result of learned behaviour. These are previously neutral stimulus, connected with unconditioned stimulus & give conditioned responses

As per the given example case :

because the early people found out about it.