!! used as default html header if there is none in the selected theme.
--- Introduction ---
The aim of this exercise is to establish (and to test) the intuition of
the student on the gravity center. This is a visual exercise: given a figure with
a number of objects (represented by points with different colors), eventually with different
weights, one should find the gravity center on clicking over the figure.
The principle is to ask the student to click without computing the positions.