Place a dry, empty aluminum soda can on the table. Build up a charge on one of the charging rods and hold it alongside the can, and you should see the can move slightly towards the rod; with a strong enough charge, you can pull the can across the table via the attraction to the rod. The charge on the rod is inducing an opposite charge on the side of the can.
Now, pick up the can, and ask the students to predict what will happen if you do the same thing with the other rod and an opposite charge. Poll them for their predictions, and the reasons behind them. Then put the can down and perform the experiment again with the other rod; you should see the same behaviour. Invite the students to discuss why this happened. The can was grounded when you picked it up, and retains no net charge on any side; the induction process is the same as before.
Note that this is the same effect that causes the ground sphere to slowly lean towards the Van de Graaff generator before a spark forms in demonstration J2-03.