The central compound of Silver's adhesive was an acrylate copolymer. But the adhesive was too weak and wouldn't stick for long. Acrylate polymers were a category of chemical compounds often used in paint, plastics and textiles. What Fry wanted was the perfect pressure-sensitive adhesive. He experimented with particles called microspheres, tiny round molecules. When sprayed onto a surface, the adhesive coating was just like the pebbled surface of a basketball. By spraying the adhesive in the form of these round microspheres, he could coat a surface that wasn't too sticky or too weak. This allowed a Post-it note to stick onto the page of a book, but it could later be easily removed and stuck again somewhere else. Conversely, normal packing tape had a flat surface, hence allowing it to stick permanently.