How do you build a waterfall that looks like it was created by a professional? Two words. Black Foam. It's so easy to work with - simply shake, point, and fill - for beautiful, fuller waterfalls. Expanding Black Foam re-directs waterflow over rocks, filling in cracks and voids to create a professional-looking waterfall. You know how it works: the rockwork is in place, but the flow of water refuses to cooperate. Grab a can of Black Foam, fill in the gaps, and you've got that picturesque "curtain effect." Also helps secure stone edging and contain gravel on bog shelves. This is no residential construction foam. It's a professional-grade, expanding cellular polymer made expressly for water gardening. Savio Black Foam contains high carbon content for increased outdoor UV resistance and long-term durability. The formulation is closed cell, and will not harden or shrink.