I don't have any grandchildren yet, but my two daughters have come up with such doozies that I can't wait to see what the grandkids will say when they come along!
When my daughters were around 6 & 3 we attended a local high school football game and sat behind a roped off area in the bleachers. Two ladies I knew were sitting inside that area. My 6-year-old asked why that area was roped off, and I noticed the two ladies smiling and glancing around at us. I explained that the area was saved for a high school reunion group...people who had graduated together who would want to sit together. She asked how long ago they graduated, and one of the ladies looked around and said, "Twenty years." On hearing that, my daughter's eyes got big and in a loud, clear voice she asked, "And they're still alive?!?" The two ladies weren't smiling quite as much on hearing that, but everyone else in the stands had a good laugh.