This is a true family story: When my son was about seven years old, my wife, son, and I were on a car trip when my son asked: "Mom, what's a flasher?". Well, being a responsible mother, she proceeded to describe this man with a trench coat. You get the picture. After the explanation, my son frowned as though he did not get a good answer and then asked: "But mom, what does it mean 'Trucks under 40 use flashers'?". I had seen the sign and knew exactly what question my son had asked, but I was so interested in hearing my wife's response that I kept silent throughout the explanation.
(From the "Rest" of RHF)