You wouldn’t know it, because it’s hiding down there at the bottom of the periodic table of elements, but it’s a prank—something a five-year-old might do—and the guy who did it was one of the greatest chemists in America. It’s pure silliness, staring right at you, right where I’ve drawn my circle, at element 94.

It says “Pu.”

“Pu” stands for plutonium, the element named for Pluto, back in 1941 the newest, teeniest planet in the solar system. The American chemist Glenn Seaborg came up with this name after his colleagues found neptunium (element 93) the year before. He and his team at Berkeley had a cyclotron that smashed particles together and so they had an incredible run of discoveries: