Our own design of the classic Cartesian Diver game includes one standing diver, one hook diver and one pressure pump bottle cap (not shown).Children can investigate the relationship between density, pressure and volume as well as hypothesize and test myriad variations. Just add your own 2L plastic bottle; your students will enjoy hours of fun and learning.With two, students can compete to provide the perfect pressure to hook one diver onto the other. Our construction is designed for durability and manipulation.We are so proud of it, that this item comes with a lifetime warranty!If any of the parts ever breaks from normal use (even normal child misuse), send it back and we will replace it free.Period.For as long as you own it.If you've ever had another Cartesian retriever, then you know how easily most of them break. There are so many possible variations and experimentations that this becomes one of the best manipulatives for practicing prediction, observation and recording, as well. GET THE HOOK! Kit...........................$5.95 ea.