Dirty tennis ball.

"But Jim, now, he knows it happens, he watches for it happening, he sees it start, he sees it finish, he licks the wound he expected, and never asks why; he knows. He always knew. Someone knew before him, a long time ago, someone who had wolves for pets and lions for night conversants. Hell, Jim, doesn't know with his mind. But his body knows. And while Will's putting a bandage on his latest scratch, Jim's ducking, waving, bouncing away from the knockout blow which must inevitably come." - Ray Bradbury

A one-legged sanderling hops after his mates at the water's edge on the beach at Bay of Lopness.