My wife used to organise this at her school each year.

This Extended Play Variation is one of the best ways to show children how hand wiping stops the spread of germs around the room.

Fill a large plastic bag with flour or cornstarch. Explain that this is a container full of "pretend germs," while reassuring children they are not real.
Have children pretend that they forgot to wash their hands and ask them to dip their hand in the mixture before they go off to the next activity.
Soon children will see the trail of "germs" they are leaving on everything they touch!
End the activity by giving each child a Wet Ones® wipe to clean off the germs they left around the room. Then give one more to wipe their hands before they go off to play... no trail now!