1.  catch on(v = verb.cognition) cotton on, get it, get onto, get wise, latch on, tumble, twig - understand, usually after some initial difficulty; "She didn't know what her classmates were plotting but finally caught on"
is one way to apprehend, compass, comprehend, dig, get the picture, grasp, grok, savvy
Sample sentences: Somebody ----s; Somebody ----s PP

2.  catch on(v = verb.change) Array - become popular; "This fashion caught on in Paris"
is one way to change
Sample sentences: Something ----s; Something is ----ing PP

catch, catch a glimpse, catch a wink, catch at straws, catch cold, catch crop, catch fire, catch it, catch one's breath, catch out, catch phrase, catch some z's, catch title, catch up, catch up with, fair catch, glottal catch, safety catch, shoestring catch