That's the lesson some local 8th graders learned.
Students at Holy Rosary Regional Catholic School in Plymouth meeting put their cardboard boats to the test.
They've spent months designing and putting them together.
The goal was to make one that would hold two students, and carry them across the pool at the Plymouth Swim Club.
As you can see, some made it, but a few didn't.
At least it's a hot day and that water probably felt good.