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Lolly half-life

By David, 5 May 2015 Activity

Someone is writing numbers on a strip of masking tape.

Number the marks on the tape.

You will need

  • Bag of M&Ms or Skittles or any lollies with writing on one side.
  • Masking tape
  • Marker pen
  • Ruler
  • Food-safe gloves
  • Large, flat-bottomed container

    Someone is holding a lolly showing the letter 'm'.

    Separate out all the letter-up lollies.

What to do

  1. Stick a long strip of masking tape near the front edge of a table.
  2. Put a mark every 5 cm and number the marks from 1 onwards.
  3. Put on the gloves.
  4. Open the bag of lollies and count them.

    A long line of lollies.

    Line the letter-up lollies with the first mark on the tape.

  5. We are going to play a game where we remove about half the lollies each turn. Estimate how many turns it will take before the lollies run out.
  6. Collect all the lollies up in your hands, and then drop them into the flat bottomed container. Separate them into two piles – those with a letter facing up, and those with a letter facing down.
  7. Move the letter-up lollies onto the table and line them up with the number 1 marker on the tape.

    Someone is pouring a handful of lollies into a container.

    Pick up the lollies and drop them into the container again.

  8. Pick up the letter down lollies, and drop them into the container again. Separate the letter-up lollies and line them up with the number 2 marking on the tape.
  9. Repeat this process, dropping the lollies into the container, and moving the letter-up ones to a marker on the tape.
  10. Once all the lollies are on the table, look at the pattern they make.
  11. How many rounds did it take to finish this activity? How close was your estimate?

    Lines of lollies that gradually get shorter and shorter.

    Keep repeating the experiment until you run out of lollies.

What’s happening?

It probably took about eight rounds before all the lollies landed letter up. You may find that surprising, since half the lollies were letter-up after the first round. The average ‘lifespan’ for a lolly in this activity is about 1.5 rounds, yet some lollies may survive four or five times longer!

This sort of process, where a quantity keeps halving, is called exponential decay. The most famous example of exponential decay is found in radioactive materials. Every radioactive substance has a half-life, the length of time it takes for half its atoms to decay.

Many drugs, including caffeine, are eliminated from our body following an exponential decay. Hours after the initial buzz has worn off, there is still a little bit of caffeine in our blood. This might explain why some people find it hard to sleep when they drink coffee in the afternoon.

More information

How does carbon dating work (video)

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