To set up this challenge, we just used the First Lego League table and tied the string to each of the lamp posts.  Then we put a large vase at one end of the string.  We're using small rocks for the pebbles.  Each team can use up to 15 pebbles.  They will get one point for each pebble in the vase up to a maximum of 10 points.

   5   For working robot
   5   Use switch in program
   5   For robot that moves across the string
10   One point per pebble that ends up in jar
   5   For robot that returns on it's own.

We put an air mattress on the LEGO table under the string, just in case any
robots fell off.  

Of course, the kids were amazing.  Two out of three teams had robots (Vine Climber and String Stormer ) that did the challenge perfectly by 2pm.  On the schedule, we don't start final runs until 3 pm!  What would they do for the next hour?

They all went and worked with the third team to try and get their robot working.  About 2:55 they were taking their robot apart.  The other teams started their runs at 3pm with two perfect scores.   The third team showed up at 3:10 with an untested robot.  On their first try, it went across the string and dropped 7 out of 10 stones in the jar!  On their second try, they got all 10 in the jar.

In other words, all three teams had perfect scores on the day when I wasn't even sure they'd get a robot to do anything!  They all came up with different wheel/gear/pulley systems to get across the string and used different combinations of sensors too.  Here are some photos of the robots.  One team's robot (Vine Climber) had the ultra sonic sensor pointing down at the ground.  They dropped the rocks when the sensor found an object (the vase) that was closer than the floor.  The vase was about 2 ft tall, so that worked quite well.  

String Stormer	String Bot
Vine Climber

Read more about LEGO NXT Mayan Adventure week.