On Thursday night, Billy Eichner brought his abrasive brand of charm to the “Late Show With David Letterman.” The two played a surprisingly difficult game called Celebrity Child or Kentucky Derby Winner during which Eichner read a series of names that could be, obviously, names of the children of celebrities (i.e., Pilot Inspektor) or names of Kentucky Derby Winners (i.e., War Admiral). Letterman is adorably amused through the whole thing, which is never more apparent than when he shrieks, “Keep going!” when the game’s clock has run out.