Albert Einstein, the real-life physicist, is depicted in a couple of episodes of the show. He is voiced by Paul Rugg.
In "Cookies for Einstein," he is the last stop on the Warners' Kid Scouts cookie-selling route in 1905 Berne, Switzerland. He refuses to buy any cookies until he has cracked the theory of relativity. While somewhat annoyed by the Warners, he is less adversarial than many of the other people they confront throughout the series. Mostly, he is extremely down in the dumps, calling himself "stupid little Einstein dummy head." Eventually, he cracks the formula with the Warners' help, by writing "acme" backwards (with a deformed "a"), resulting in the formula e=mc 2. (As was later noted in "Please Please Please Get a Life Foundation," this is actually the energy-mass conversion formula, not the relativity formula).
He is in the row of heads Yakko plays like a xylophone in the main title at the beginning of every episode.