The lighthouse paradox is a confusing idea about infinity. To explain it, imagine that a lighthouse is turning round and round in circles, and the light from the lighthouse can travel infinitely far away. What the paradox asks is "Where is the edge of the beam of light, if it can travel forever?" There isn't an edge, so the paradox is asking how, if there is no end, can the light have a beginning? It's like trying to measure something that doesn't have an end.