Pepper spray is back in the news in California. Students at Santa Monica College were pepper sprayed by the police during their ultimately successful protest of higher fees for the most popular classes. Other high profile clashes with the police have included the tear gassing of Occupy Oakland protesters earlier in the year and the notorious misuse of pepper spray by a University of California police officer against students at UC Davis.

Every time one of these incidents hits the news, the scientist in me wonders what the difference is between pepper spray and tear gas. My own experiences with tear gas in protests in Oakland shed little light on the distinction.