That means you can't have an electron approach another electron - they will repel one another. And if any particle meets its antimatter counterpart, both will "annihilate."
Per Hall's Website: "An electron has a negative charge. A muon neutrino has no electric charge. ... Opposite charges attract, like charges repel. Matter and antimatter will explode in a burst of energy." You get the idea.
Did I mention there are more than 100 levels? As with any game, as you move up, the game gets harder. You might want to brush up on your Standard Model.
Not surprisingly, Hall used to a physics grad student, and did a stint at the Museum of Science in Boston doing science outreach before branching out into the world of game design.
"My goal is the make things that bridge the gap between the worlds of simulations and games," Hall claims, and he thinks these games like Agent Higgs are a great way to introduce players to some pretty abstract physics concepts. "I want to combine the value and rigor of simulations with the learning curves and motivation of games."