My son (just turned 5) is only MG, if that, but I love these stories and I'm going to join in...
When DS asks out of nowhere: "Mummy. If I was standing on an exoplanet, could I see our sun twinkling like the stars we see at night?"

When DS comes across some Roman numerals in a book he's reading in the car and asks about them. I give one brief, distracted, description and he is instantly able to work the rest of them out without skipping a beat.

After explaining synonyms, DS comes up with many on his own, then asks "What about numbers? Is there a word like "synonym" for sums? Is 6+1 a synonym of 3+4?".