Science project question...?

My cousin's 8th grade science project teacher did an experiment where he took a box and there was a funnel on top of the box with a tube connected to the funnel. The tube ran through the box and out the side. He poured 1000 ml. into the funnel, but 1700 ml. came out of the tube. He said that there was only one item in the box. He also said it was a simple answer. anyone have any ideas? tnx for any answers!