On our recent vacation, my wife and I visited a nature preserve off the coast of Argentina where we had the chance to swim with sea lions.Even though they were half-grown pups, some of the sea lions were as big as I was. They were curious, friendly, and playful. Like many mammals, their play takes the form of gently nipping at each other. They seemed happy to include human visitors in their fun and games. We could reach out and touch them, and they would respond.