易

Top sun - the indexing radical. One reading of 易 takes the picture as the sun emerging from clouds, where light shifts moment to moment, giving the sense of change. From change came easy (a thing that changes hands easily) and exchange. Same sun family as 明, 时, 早, 晚.

Bottom strokes depict slanted rays streaming from the sun above - not the negation 勿 but a graphic of flowing light. Sun-with-rays gave the early sense of brightness shifting into shade, hence change and easy. An older reading takes the whole glyph as a chameleon, an animal famous for changing color - either way 易 captures mutation.