A demonstration I use while teaching static forces is to have two students hold the ends of a long string with a spring scale (force meter) attached to each end.  I ask the students to each pull with 10 Newtons (as measured by their spring scale) and try to hold it constant.  I then ask the class “What is the force on the string?”  They invariably say “20 Newtons.” 

I then give the two students another string with a spring scale in the middle (actually 2 shorter strings with a spring scale already attached in the middle) and ask them to again pull with a constant 10 Newtons.  I have a third student read the force meter in the middle, which reads 10 Newtons.  This is a hard concept for the students to understand until they see it for themselves.