Joey has always done well at language development, like rapidly picking up new words and easily following new grammatical concepts. One thing he latched on to pretty quickly is the meaning of "past tense".
When kids are learning to talk, they quickly learn that if they want to say that something happened in the past, they just add an "ed" to the end of the word. It works 99% percent of the time, so they figure it's a safe bet. With Joey, every time I heard something like "I eated that sandwich", I made a habit of pointing out "The past tense of 'eat' is 'ate'". I didn't really spend any time explaining what a "past tense" is or demonstrate the correct usage of the new word or anything. I would just throw it out there and see if he caught on. He did catch on really quickly and has a much better usage of all of those irregular declensions than other kids his age (if I do say so myself).
A couple of months ago, we went to Costco for the semi-monthly stocking up. Right after we finished checking out, Joey said, "Wow, look at all these things we buyed!" I responded, "Yeah, we bought a lot of stuff." Joey looked down and muttered, "That's right, the past tense of 'buy' is 'bought'" in his best "Please, Daddy, don't hit me for getting it wrong" voice. I was mortified, but quickly looked around and was relieved to see that no one had overheard this little exchange.
I was reminded of this yesterday when Joey walked up after having just finished a drink and told me, "Daddy, the past tense of 'full' is 'empty'." I could not argue with that logic.