"That's it! Let's just make the number up!"
"Can we do that?" asked John.
"Why not?" said Betty. "Let's just make the side the right length so the square has area exactly three square metres, and call it the square root of three."
"I see," said John. "So if means the square root of 3 then = 3".
"Exactly," replied Betty.
So they tried it.
And it worked!