Helping with Math homework

So...I'm helping Desmond out with his math homework. The question is simple:

3 + 4 = ?

Desmond looks at the question and then asks me, "Dad...what's 10 minus 3?

I say, "Seven." 

Desmond writes 7 as the answer to 3 + 4. 

Correct answer right? 

But, wait...I say, "I don't get what you just did.  That's the right answer, but I don't understand why you asked me what 10 minus 3 is."

"BECAUSE!" Desmond says, hating to have to explain something to a simpleton like myself.

"Because why?" I ask.  "How did you know that 3 + 4 = 7 by asking what 10 minus 3 is?"

"Easy!" he replies. "Because 4 plus 6 is 10.  So, all I needed to do was take 3 away which makes 7.  That's how I got it."

Huh?  What the heck just happened?  Everything he just said is correct.  But...what does 10-3 have to do with 4+3? 

My head hurts.