Content warning - not even stealth boasting parent alert.
DS aged 3 ½ correctly and completely out of the blue used the word 'symbiosis' today.
We were on a very bumpy train and he wanted to drink from a bottle of juice. I said I would help him hold it so it didn't spill.
DS: "But I can change my top if it gets wet."
Me: "I would rather you keep it dry as it would be more fuss for me to have to change it."
DS, letting me steady it "Symbiosis."
Me: "That's a very clever word. Do you know what it means?"
DS: "Yes, it means two things working together."
Me: 

I have no idea where he learnt that from!