learning about density

It floats, but when it fills with water, it starts to sink. How can we fix it? "With tape?"

better solution found to stop the filling up, and then "it needs a 'repellar' you know, a thing on the back to go round and round." luckily the sour cream container was an acceptable solution.

*Matthew chose his boat parts fro the recycling bin after a D&D night... I myself would have went for the empty two liter, but Matthew likes the color green.