This puzzle has a light-colored helix in front. You can insert segments into this helix. A thin red line at the right end of the helix is a constraint. Constraints work like bands, pulling things together. Unlike a band, you can't delete a constraint. Design puzzles use constraints to keep parts of the protein from moving around too much. Constraints often appear in puzzles like this one, where only part of the protein is available to design. Other design puzzles let you build an entirely new protein from scratch, and don't have any constraints.