I was painting the other day after eating lunch from McDonald's.
I was using the cup to hold my water for rinsing the brushes out and the top as a palette.
I was using the side that faces toward the drink to hold the paint.
30 minutes into painting I had to run to the store real quick.
Not really thinking about it I put the top back onto the cup filled with water.
When I came back a hour later the paint was still usable, the water in the cup acting as a wet palette.
While it does not last as long as a true wet palette it does help during those situations where you
get dragged away from your painting for short periods of time.