Tape a large piece of paper on a wall at your eye level. Ask two or three Scouts to come up and make a mark on the paper with a marker as high as they can reach.
Thank them for their effort and allow them to return to their seats. Tell the troop that we can all usually do better than our first effort. Remind them that you asked the Scouts to reach as high as they could. Ask the same Scouts to come back up and see if they can do better than their first effort. (It never fails that they will always reach two to three inches higher on the second try.)
This is a good opportunity to emphasize doing one’s very best, and to give every project one’s “second effort” on the first try.