Some are packed with sand, others with marbles, and others with pony beads. Not only is their mass different, but so is the volume of each instrument. These factors affect the instrument's density, and therefore, the amount of water they displace. Ultimately, this determines whether something will sink or float. Taking all this into consideration, second graders are thinking about a real problem an ocean engineer might face...how would you get a floating submersible to sink down to the ocean floor to collect data? And how would that sunken submersible be able to return to the surface and float once it got the data it needed? Hmmm....what a problem to have!