A Blonde was on railroad tracks at a crossing skipping rope, with a large duffel bag lying alongside the tracks.  As she skips, the Blonde says, over and over, "12, 12, 12...."  

A Brunette walks up to her and says, "You're doing it all wrong.  Let me show you."  The Brunette takes the rope from the Blonde, who steps aside for the demonstration.  The Brunette then skips rope and counts out in the usual serial order, "1, 2, 3, 4...," when suddenly a train roars through, wiping out the Brunette.  

After the train passes, the Blonde reaches into her duffel bag, pulls out another rope and walks onto the tracks to begin skipping again.  "13, 13, 13, ....."