TOPIC:

A picture is supported by a string from two pegs, as seen in the photographs below from the left and right sides respectively. The interesting thing about how the support is strung is that if you pull the string off either pin the whole thing comes off both pins and the picture falls.

See the Physics Question of the Week for 11/3/2014 for an example of how this demonstration can be used in class interaction and discussion: lecdem.physics.umd.edu/question-of-the-week-archive.html