A reader requested an explanation of the “knuckle knock” or “fist bump” by Michelle and Barack Obama. In general, it is a Purell moment of affection or acknowledgement. In their case, it looked hip, especially with Michelle’s “thumbs up” gesture afterwards. Another celeb who practices the “knuckle knock” is Tiger Woods: Watch him after he or another has made an extraordinary shot. Even Mister Monk can join in the fun without fear. It’s harmless, E.D.