If you hope to come back and find some left in your mug that is.
The other day I was sitting on our little front porch enjoying my morning coffee before the boys woke up. Eventually Ethan joined me out side with his sippy cup. For some reason I had to go inside for something and left my coffee cup sitting out there. I came back to see that the little stinker had unscrewed the lid of his cup and poured the entire contents of my coffee mug into his sippy and was trying to screw the lid back on his, now full of MY coffee, sippy cup!
Here he is trying to look all innocent.