From the site (http://www.teachwithportals.com/):

Welcome to the new, free educational collection of puzzles and teacher-created content from Valve’s best-selling game, Portal 2, an engaging 3D puzzle-solving game. Based on Valve’s technology, the Portal 2 Puzzle Maker takes place in an environment with realistic physics – a playground rich with opportunities for educational fun.

Seems to be physics or problem-solving.