Arty Phase

This piece of art work was produced by a student at Westwood International School and it is placed in a traditional thatched "Lapa" used by the art department for outdoor art lessons. 

It is aptly located next to the science department, who can make use of this piece of artwork to remind students of a very important piece of theory.

Which piece of empirical scientific evidence is unwittingly displayed by this piece of artwork?

Gaborone, Botswana, 13/5/2004