This note contains spoilers for the book.

One of the really popular ones in fandom. And all the cat lovers know just what the title refers to (and I think people who don't use the phrase to describe cat behavior).

Has time travel, though, always a disadvantage. Going forward via suspended animation is semi-reasonable (lucky and convenient that the first experiments were done on cats, so Pete could go with him!), but he has a real time machine going backwards, and hints that Leonardo Da Vinci was an American. As later in Time Enough for Love, he asserts that paradox is impossible, but nobody actually tests it. In fact it's startling how casually Heinlein universes receive time travel; nobody seems to think much about ways to benefit from it, or abuse it for that matter.

This is one of his most engineering-centered books. The protagonist is one of the early players in home automation, founding Hired Girl and making something that's darned close to the Roomba, except that it's smart enough to pick up things too big to be vacuumed and put them in a tray.

Also has a bad romance, and a good romance. The good romance violates rules on age-appropriateness, except that before it's formalized or they do anything sexual the girl has grown up without the guy around at all. And it's her idea, mostly, anyway. I think this is the first time this comes up in Heinlein's work. It pops up again in Time Enough for Love, and those two appearances get a lot of people obsessing about it.

I think, if you have to mess with time travel, you should just write "All You Zombies" and be done with it.