We filled up a tub of water and the boys made some lego 'vehicles' that could float in the water. They tested out their designs and kept improvising their designs to make them float in water. Unknowingly, they are learning science and having fun at the same time.
The blue lego on top didn't float so well, so S thought about it and decided to fix to flat "wings"
Fixing the large flat "wings"
It floated very well and S was pleased with the result