Teach: Electric fields
Aims text Considerations Electrostatics demos are notoriously problematic, you ask the class whether two objects you just charge oppositely have the same charge or different, the class all says "the same", you triumphantly say,. "No, you're all wrong". Hold each in turn next to a charged electroscope and show they were right. Get out of that one. It's often due to the atmosphere and can be explained but not easily....