Stupid Guest Tricks

I remember one strip that had Billy wandering out of his front yard from the house, and a little girl doing the same from the neighboring house. Both Thel and the girl's mom were telling their respective husbands that their child had just asked them who would they marry; at the bottom was the announcement of Billy's marriage.

As to the Thinker quote, I recall seeing that in a Dennis the Menace strip, probably before the Family Circus strip.

The "Thinker" gag is probably one that has been used by many cartoonists, but the Family Circus strip that references it is in a book called P.J.'s Still Hungry!