Once a plant or animal dies its carbon-14 content gradually decreases.
All you need is a bunch of pennies (or any other type of coin) and a clock or stopwatch. When the 30 seconds have passed, dump your coins on a table and remove all the 'heads'. The 'heads' represent 'decayed' atoms, and they will no longer be part of our sample.
We are going to assign a half-life of 30 seconds to this made-up radioactive element. Write down how many remain, and shake the remaining coins for another 30 seconds.
Scientists measure how much carbon-14 is left in a sample, and they are able to estimate how many half-lives it went through.
This will allow them to get an approximate idea of how old the material is. However, many don't pose much of a threat because they have such long half-lives.
This continues for the rest of the movie until all of your popcorn is gone.