I said this to my Algebra Teacher once, she did not get it. That's okay, I didn't get Algebra either. My Geometry teacher had a far better sense of humor. Algebra never make sense until after I took Geometry any way.
Do not eat coffee with knife and fork you will make a mess. We haven't seen a sign like this in Vanuatu but if we do we will be sure to share the note so others know what the sign means - Tanna Coffee team.