-소 is a variation of -세요 (하소=하세요, 가소=가세요). So 미소 can mean 미세요 (imperative of 밀다: push), and 반대말 of 밀다 is 당기다(pull). So one would think of 눈물 or 울음 for 반대말 of 미소, but the answer given is 당기소, the imperative of 당기다 :-)