As others have already said. Thats a needle valve from a carburator. Theres no question about it. It also isn't something that falls out. Someone did some lawnmower repair and either was annoyed when they lost it or just pitched the old one in your yard when their parts kit came with a new one. There's probably not much point in saving it though because they're dirt cheap and easy to get. Whoever lost it has almost certainly replaced it by now because without that little needle the carburator won't work and will start pouring gas everywhere.