Follow these instructions to find out how popcorn relates to carbon dating.

## Safety

This activity requires the use of a microwave. Ask an adult to assist you.

## You will need

• microwave
• four bags of microwave popcorn
• large bowl
• pen and paper
• friend.

## What to do

1. Popping popcorn is a little like the process behind radio carbon dating

Place a bag of popcorn into the centre of the microwave’s turntable on a low setting and set the timer to five minutes.
2. Note when you hear the first popping sounds. Write this time down next to the letter ‘A’.
3. Wait a further ten seconds. Press ‘stop’ on the microwave. Wait a further five seconds and open the door.
4. Empty the bag into a bowl and wait a minute for the hot unpopped kernels to cool before touching them. Write down roughly how many of the corn kernels have popped and how many haven’t. Calculate this as a percentage. Helpful hint: divide the number of popped kernels by all of the popcorn kernels in the bowl (popped and unpopped). Multiply this number by 100 to get a percentage.
5. Repeat steps 1 and 2 with a second bag of popcorn. Wait 30 seconds after the first popping noises before you press ‘stop’. Count how many kernels popped and calculate this as a percentage.
6. Repeat steps 1 and 2 with a third bag of popcorn. Listen to the popping sound, and watch the clock. When no popcorn has popped for ten seconds, write down the time (minus those ten seconds) to note when the popcorn stopped popping. Call this time ‘B’.
7. Give the fourth bag to a friend and ask them to choose a time between ‘A’ and ‘B’ without letting you know what it is. Ask them to microwave the popcorn on the same low setting for that amount of time.
8. Open the bag and count the popped popcorn. Calculate this as a percentage. Compare this percentage with the other numbers to estimate how long it was in the microwave for.
9. How close is your estimate?

## What's happening

Like all types of corn, the grains (or 'kernels') of popcorn consist mostly of starch, water and oils surrounded by a thick, waterproof husk. When popcorn is heated to boiling point, the water inside the kernel turns to steam and the starch gelatinises, meaning it combines with the water to form a thick, flexible material. The pressure eventually becomes so great that the kernel ruptures, releasing the steam and allowing the gelatinised starch to expand rapidly into a fluffy (and delicious!) ball.

Not all kernels are identical, however. Small differences in the amount of moisture, the amount of starch and the strength of the hard outer shell mean some kernels will require a little more heat to pop than others, and others a little less heat, ensuring they don't all pop at once.

## Applications

In some ways, popping popcorn is a little like the process behind radio carbon dating. The different ratios of popped and unpopped kernels tells you how long the popcorn has been in the microwave. By comparing them, you can work backwards and 'date' the time the popcorn has been cooking for.

Instead of popcorn, carbon dating uses something called a carbon 'isotope' (EYE-so-tope). Isotopes are atoms of the same element (meaning they have the same number of protons) but have different numbers of neutrons. All carbon atoms have six protons, but carbon-14 has eight neutrons while carbon-12 has only six.

Carbon-12 can capture extra neutrons in the upper atmosphere, which changes them into a different isotope. This is why carbon-14 can be found in carbon dioxide in the surrounding air. Since plants absorb this in photosynthesis, living things always contain a small amount of carbon-14.

Over time, carbon-14 breaks down, where a neutron turns into a proton, making the carbon nitrogen. Like the popped popcorn, comparing the amount of carbon-12 and carbon-14 in something that used to be alive will indicate a length of time. In this case, it's the amount of time since the organism stopped absorbing carbon-14.