When DS7 was 18 months or so, we had a book of opposites we read him once in a while. Drawings of a duck and a goose showing up/down, fast/slow, heavy/light, etc. We stopped reading it to him because it seemed to upset him. It went into the "to be donated pile - which we only rarely get around to actually donating.

About a month ago we were sorting the pile, and came across that book. He started paging through it, reading it to me. Then he got to the drawings of "happy/sad" and said, "I never did like that page."