Prepare the strips for the activity:
Add a smear of vaseline to the end of each strip. Make sure you use the same amount each time and add it in the same spot.
Push a penny into the vaseline on each strip.
(Adjust the amount of vaseline so the penny sticks, but does fall off the metal strip within a minute after the experiment starts - you do not need much).
Add just-boiled water to the coffee cup, then simultaneously add a metal, plastic and wooden stick to the water with the pennies pointing upwards. Make sure they are sloped outwards by the same degree, so that this is not a variable in the penny falling off. (Penny has just fallen off the aluminium strip in the first photo.)
Record on worksheet (see attachment) which penny falls off first, second and third. Some pennies may stay stuck for longer than you want to run the activity, but make sure at least one has fallen off.
The metal should release the penny first, but students' experiments may vary.
Record the class results, to find out what happens most of the time. Should be the metal conducts fastest, and the plastic or wood are last to release the penny. See second photo for a class data set.
Discuss the mechanism:
Heat moves up the strip by conduction. Once the heat energy reaches the vaseline it melts it and causes the penny to fall off. The different materials conduct heat at different rates: metals conduct heat the fastest, wood and plastic much slower.
Discuss the molecular mechanism with older students:
The hot water gives energy to the strip where it is touching it. The energy from the hot water makes the molecules of the bottom end of the strip move faster. The heat spreads up the rod as the faster molecules at the end of the strip bump into adjacent molecules and give them energy too - this is heat movement by conduction. When the heat energy reaches the vaseline it melts it (the molecules of the vaseline move faster as they become a liquid from a solid). The melted vaseline can no longer hold onto the penny, so the penny drops.
The movement of heat when molecules transfer energy between each other by colliding with each other is called “conduction”.
Some materials, e.g. metals, have molecules that can transfer heat energy to each other quite easily, so conduct heat well. Other materials, e.g. wood and plastic, have molecules that are less good at transferring energy, so do not conduct heat as easily.