Plato is lecturing his students on the nature of change and how nothing is ever constant.

To drive his point home, he asks his students to give him an example of something definite.
"Master," says the first, "the leaves on the trees are definitely green."
"Not true," answers Plato, "for they can appear brown, yellow, or orange in the autumn months."
"Master," says the second, "the sky is definitely blue."
"Again, not true, for it can appear red or violet in the twilight hours and even black at night."
The third student thinks for a second and asks, "Master? Are farts lumpy?"
"Well...no," answers Plato.
"Then I definitely just shit my toga."