Have the student stand up with one of their arms outstretched straight to the side. Tell them that you are going to apply pressure on their hand and try and push it down. Their job is to keep their arm up in the horizontal position. Ask the student to think about something horrible that has happened to them. When they have said they have thought of something, give them the instruction to "think, think, think about it" as they are doing that apply the pressure. Their arm will easily go down.
Now do the same activity, but instead of a negative event, have the student think of the most wonderful thing that has ever happened to them. Again tell them to "think, think, think, about it". This time the students arm will remain strong, straight, and immovable.