window to weight gain 

When the paper wrapping around a fatty item of food (traditionally, a fish-based item such as fish and chips or a fish sandwich) turns translucent due to having absorbed a large amount of grease.

"If you're not sure about something, rub it against a piece of paper. If the paper turns clear, it's your window to weight gain."

window to weight gain by Zerotime January 22, 2009