Hydrophobic sand or said in plain English sand that does not get wet. Check out the video filmed by Eric Olson of FeedBurner. If you're interested in buying some contact the vendor https://www.inventables.com/technologies/waterproof-sand
One of our motivations for launching the new Inventables Marketplace was to end cold calls. In our view cold calls are ruining the world. We hope our new website puts an end to cold calls. The reason we think cold calls are bad is they interrupt productive work. Most engineers that work in product development wish they had more time to work on designing their product. They often find themselves in back to back to back meetings and when they finally get back to their desk the phone starts ringing. We hope that Inventables Marketplace will be so effective at connecting vendors with new customers they will spend time educating the world about their products by creating and updating product profiles like this one for Overmolding with Plasma Treatment rather than cold calling prospective customers and interrupting their work.