A lovely stroll along the path by the river today to my favourite park (which the children have christened "Mary's Park", much to my delight). On the way, we pass a wooden fence, painted in broad black and white stripes. We've always passed it, but today Thomas notices for the first time, and asks why it's there. I explain that because the road ends at the river, the fence is to warn drivers that they can't go any further straight ahead. Trying to let him discover the safety concern for himself, I ask, "What would happen if a car didn't turn, Thomas? Where would it end up if it went straight through?" He doesn't miss a beat: "Wet."