What is a Fraction?

courtesy Math Smart Junior
by Marcia Lerner

When the pizza came out of the oven it was beautiful and steamy and smelled of tomatoes and garlic.  Everyone was hopping around the kitchen because they couldn't wait to eat it.
"Okay," said Tom, who'd put on a big white mushroom-shaped chef's hat, "Step aside, step aside...it's time to add the mouse heads.  Whoops! Only joking. Now, let's cut this thing."

Here's the first cut.  He cut straight down the middle of the pizza, like this:

How much of the pizza is each piece?  One-half written as: