Passing aliens see a man sitting in a boat rowing down the river singing, "Old man river, that old man river...
Just as an experiment, they beam him up, remove half of his brain and beam him back down again. Straight away he starts, "Old man river...
This has the aliens scratching their heads and wondering how he can possibly do that, so they beam him back up and remove another quarter of his brain, then beam him back down. Sure enough, he starts rowing again and singing "Old man river...
By now, the aliens are pretty puzzled as to how a man with just a quarter of a brain can still be doing this, so they beam him back up again and remove all of his brain - even scraping the inside of his skull to make sure there's none left. Then they beam him back down again, and he starts rowing, singing, "Ferryyyyy cross the merseyyyyyyyy...