So when someone starts whistling, you spontaneously start peeing?

My condolences.
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At the track, after a race, all the horses in the money are drug tested by way of a urine/blood sample.

So from the first time they arrive at the track, whenever the horse urinates the horse’s groom starts whistling so that after a half-dozen or so times the groom can just start whistling and it will trigger the recent, environmentally learned instinct and the horse will spontaneously begin to urinate.

Speed things up after a race, time is money ya know.

Anyways, that’s where the saying, “piss like a racehorse actually comes from”.