How to: explain the Doppler effect to a kid

When I was a 6- year old I visited my uncle's dairy farm near our home town. While there a whistling train passed nearby. My sisters and I noticed that the pitch was higher when the train was approaching and had a lower frequency when getting away from us. Sister Miyaray hypothesized trains might have two whistles, one to let people at a station know the train was approaching it, and other to announce the train was departing. The only inconsistency: there was no train station in sight.

Later on the same day, I fell into a deep hole in the ground and had to be rescued. My parents concluded I had been frightened to the point I could get ill from "susto" and took me to the traditional healer at the market to be prescribed herbal teas.