A simple but great idea

Vincent loves to ride his tricycle around now that his legs are finally long enough to reach the pedals.  He does a great job of riding it in the gravel.  Sometimes he rides it from the shed to the house and parks it right in front of the door.  The walk way to the house is a sloped a little and sometimes his tricycle rolls backwards.  I found out his trick to keeping it from rolling away: 3 pieces of egg shaped chalk.