The fourth most popular move.
1. e2e4  1... c7c6  2. d2d4  2... d7d5  3. f2f3 [%c_effect f3;square;f3;type;Interesting;persistent;true] This move is a popular sideline in the Caro-Kann Defense. Very strong players choose it from time to time. 3... e7e6  4. b1c3 