The Pizza Diet

Here's one for my dial-up friends (not friends I make by dialing up numbers on the phone, that would be weird. Friends who don't have broadband). No photos, and only a small amount of text. A very small amount of text actually - just enough to ask one question.

Why, after weeks days of eating boring healthy balanced meals and watching my weight stay exactly the same, do I find that the morning after having eaten a rather large and delicious takeaway pizza, I have somehow shed 9lbs in the night?

Tia