My favorites are rather simplistic.
"In the morning I crawl, in the afternoon I stand and walk on two legs, but in the evenings I am back to three..what am I?"
"Human's spend their lives trying to capture me, often breaking me. When I am broken I still work, when you can touch me then my work is done..what am I?"
Both are very simple, and only took me a few moments of pondering back in high school to figure out. But both answers kind of speak to me in a weird way