Go to a party supply store. Get a gross of round latex balloons. (The mylar ones won't work for our purpose.) Mix and match colors, logos, whatever, as long as they're all more or less the same size. If it's near Valentine's Day, get red, pink, and white ones, so you can surprise your sweetie who you'll convince you've done this all for her. While you're there, get a keg of good beer, unless your friends are used to cheap beer, in which case you should get a keg of Milwaukee's Best. (If your friends are real mutants, get Blatz.)
Call up your friends and tell them about the keg, but not about the balloons. If you don't have friends, call up the local chapter of Sigma Nu, who'll be willing to do anything for beer. When they show up (and this is key), offer up the beer only after they have blown up the balloons. If you let them drink before, some damn fool is likely to suck instead of blow, and then you'll spend the rest of the evening in the ER while interns are performing open chest surgery to remove the balloon from your friend's lung.
So, the next day, after you've cleaned up the inevitable broken glass, cigarette butts, guacamole droppings, and mysterious stains on the couch, take out the now inflated balloons. You should have a gross of balloons, minus the ones your friends popped to see how high they can make each other jump, and the one that someone peed in on a dare to see how full they could get it. (Now you see the importance of having the balloons inflated and put away before the drinking starts.)
Rub some of them on your head to give them a static charge and toss them among the others on the floor. They should clump together due to the charge. Take three or four from the edge and hold them in your hand.
Now, visualize the balloons on the floor as being atoms in a desktop. The surface of the balloon is the electrically negative field-of-effect of the electrons. The actual electrons are somewhere inside, and the nucleus is near the center of the balloon. The static charge you've applied to the balloons are the molecular bonds (ionic or covalent) that hold molecules together and to each other. The balloons in your hand are part of your virtual hand, and are held together by similar forces.
Now, try pushing the balloons in your hand (which we'll just call your "hand" for the rest of the example) through the balloons on the floor, which we'll call flooroons, partially to distinguish them from the ones in your hands, but mostly because it looks funny. If you just lay your "hand" gently on the flooroons, they'll rest there in equilibrium. Even though there is space between and inside the flooroons, the gaps aren't wide enough for your "hand" to fit through, and the space inside of the flooroons is bounded by the balloon skin. If you try to pull them away, they'll stick a little bit (if they've a static charge) and one or two might stay if you don't hold them tight enough. If you push them harder, they'll push the flooroons apart and go in between, breaking the bonds between the flooroons.
If you keep the flooroons contained, say by dragging some of your still-passed out friends and using them to make a barricade around the flooroons, and then force your "hand" into the flooroons, the balloons (both in your hand or on the floor) will start to deform. The harder you push, the more they deform, until one of them gives way and pops, which will no doubt wake up your friends, who are wondering why they are sleeping on the floor, what in the heck they are doing surrounded by balloons, and why you are standing above them with an illuminated look on your face.
It is vitally important at this point that you not explain what you are doing. However companionable or accomodating your friends might be under normal circumstances, they are unlikely, in their current state, to be impressed by the demonstration you have performed or your explaination of it. Instead, you should promise in oblique terms never to talk about what happened the previous night and promise to keep all photographic evidence to yourself if they help you clean up the mess.
You will now have demonstrated why objects can't pass through one another, and your friends will think that you throw really wild parties. Yet another triumphant victory for science.
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