To explain this joke to people who didn't grow up in Germany: when German children play hide and seek, the seeker counts to a prenegogiated number, and then yells out "Eckstein, Eckstein, alles muss versteckt sein" – "corner stone, corner stone everything must be hidden", before starting their search. The Eckstein is there for rhyming purposes. But the -stein Epstein is pronounced differently from the German word Stein, so the "sein" needs to be misspelled in order to accommodate.