Off the top of my head I can't help you with penthouse (though someone here will do that, never fear!) but if you get from "pend" (to weigh) to "penser" (Fr. to think, to weigh things mentally) to "pensee" ( "thought" with an accent aigu, which I don't know how to make on my keyboard) it's just a little hop to the Anglicized "pansy" - ta-da! Which is why pansies are for thoughts in the language of flowers.
Welcome to AWADtalk, ginax.