The "-ology" books? My parents kept my set of those in a box for however many years - at least 15 - and they've been passed on to my niece and nephew.

I am not going to lie, they still capture my attention the same way now. And I still remember the answers to all the riddles in Dragonology.

It's kind of like half way between angry and happy. Can't you just pick an emotion? /s

Not really so far as I understand it. "Bipolar swings" are more commonly on a week-month timescale.