Foldit design projects proceed as a series of 'puzzles', which are individual competitions lasting about a week. Each puzzle in a series can have a different starting molecule, protein context, scoring details, additional objectives, and tool configuration. Players use the tools available to modify the molecular system and compete to have the best 'score' in each puzzle . In small molecule design puzzles, the player is presented with the 3D structure of a starting small molecule docked into the desired target protein binding site, with the goal to optimize the ligand affinity and placement within the protein (Figure ). The user can use a set of tools, the primary one being the 'Small Molecule Design' panel (Figure ). This panel includes functionality for the player to add atoms, change element identity, add or delete bonds, change bond order, and add various pre-defined fragments such as phenyl or carboxylate groups. All changes to the molecule are instantaneously checked for basic chemical feasibility prior to acceptance, and players are informed of grossly unphysical molecules via pop-up messages.