Place a cross on a card under a tall beaker. IAsk your audience to put their hands up if they can see the cross. With a bit of luck, most should. Gradually fill the beaker with water, asking the audience to put their hands down if they can no longer see the cross. Provided the beaker is tall enough and no-one is standing too close, all hands should now be down!
These simple spinning tops can create some very interesting effects.The three on the rightof the picture are ‘strobe disks’. They give interesting rotating and counter-rotating effects under fluorescent lights as they slow down. This is because the lights flicker slightly fiftytimes per second. The effect is similar to that which makes wagon wheels in old western films appear to be rotating backwards at times. In that case, the frame rate of the film provides the strobe effect. They don’t work with LED lights unfortunatly
These spinners are made from DVDs with a marble glued in to the hole as a bearing, and a plastic bottle top glued to the top by which to spin them. You can also heat a coin and insert it through the hole as a pivot (care!). Templates for the disk patterns are readily available online.
Pepsi v Diet Pepsi
What happens when you drop a can of fizzy drink and its diet equivalent in water? Can you explain it?