Einstein's Coffee: Anticipating Relativity

For students of 11th to undergraduate grade.

It's rumoured that Einstein got some of his early ideas about relativity by watching coffee grounds in his cup.  When he stirred the cup, the grounds went to the center of the cup.  He thought this was odd, because centrifugal force should push the grounds to the outside of the cup.  

Start your physics class by offering coffee to the students.  Make sure to leave some grounds in the cup.  If you were ambitious, you could even find some early-twentieth century cups etc. and make it the old-fashioned way.  Ask students to stir their coffee and report what they observe.