By “ghosts” I mean unseen entities that influence the material world, and I certainly do believe in them. I’ve never actually seen any ghosts. But they are there!
Tell the kids you are going to show them how to summon ghosts.
Find a bottle with a narrow neck, like a wine or soft drink bottle. A penny or dime should just sit in the rim of the bottle’s mouth. If it falls through, try another coin or a different bottle. In these photos, a penny sits in the mouth of a wine bottle and a dime in the mouth of a soda bottle.
Show the bottle—without the coin—to the kids and ask if they can see anything in the bottle. Unless whoever had the bottle didn’t finish its contents, their answer should be no, the bottle looks empty. Remember, kids equate air with "nothing". That is they do until they see physical evidence that suggests otherwise.
Tell the kids it takes time to summon the ghosts and that they require conditions to be very cold (so we know they’re not from That Place) and put the open bottle in the freezer compartment of the refrigerator.
After you’ve had some cookies and milk, have the kids take the bottle out of the freezer. Tell them to set it on the table.
Have them place the coin in the mouth of the bottle, and with an eyedropper or a finger dipped in a cup of water, seal around the rim of the coin with drops of water. And wait. It takes time for ghosts to appear.
When they do appear, their presence will be made known by the coin popping up, and then falling back down into the mouth of the bottle. In this video, note the bead of water around the coin, listen for the "clink" as the coin falls back, and watch the process repeat several times.
The coin seen in the video “popped” continuously for half an hour. The interval between pops lengthens with time. A good measurement activity for the kids to do is to repeat the process and have them measure, record, and graph the data. If you would like a generic data table and graph, email your request to firstname.lastname@example.org and I will send you a PDF file.
Some science educators will take issue with my framing this activity around something as unscientific as ghosts. But they fool themselves if they think kids believe the stories about atoms and molecules and neutrons and electrons and all of those other invisible particles that fill our elementary science texts. Worse yet, these “atoms” and “molecules” are simply pronounced (with no real evidence) to be “real”. It’s easier to believe in ghosts. Nothing is lost—and much believability might be gained—by calling these invisible particles what they are, unseen entities that influence (and actually make up) the real, material world.
The moral of this story is that something unseen is there, in the bottle, and whatever it is, it can push up the penny. Let the kids talk about what they have just observed and guide the conversation but stay away from technical explanations. If they know and believe that force is a push or a pull on an object, then they have enough to think about trying to figure out what could be invisible and push up the coin.
Later, when the kids are asked to learn about molecules and the kinetic theory of gases, they will remember the "ghosts in a bottle" you showed them and have a real-world experience (other than something read in a book) to build on.