I learned this trick at Happy Hour a few years back and have used it in my science class as an inquiry lab ever since. I just don't tell my students where I learned it!
This lab does require matches and involves fire so please use caution. Adult supervision is required!!
I suggest reading "Step One" before watching the video.
Step 1: "Water in the Glass"
This is the perfect science experiment to get kids thinking. I use this experiment to introduce the steps of the scientific method and to spark an interest in science related topics for my 7th graders. Because it's an inquiry lab, you might want to figure it out on your own. If so, do not watch the video or move on to the next step. If not, click through the steps or watch the video to learn a really neat trick. To understand the science behind the experiment, an explanation is on the final step and in the description below the video. You can also check out some of the brilliant comments by members of Instructables in the comments section!!
Here is how I pose the question to my students...
"How can I get the water from the plate back into the glass without lifting the plate?"
These are your materials:
A dinner plate, a drinking glass, water, a lime wedge, and 5 matches
Step 2: Procedure
As I'm handing out the materials, I have my students write at least two possible solutions to the problem before discussing as a lab group. After all materials are handed out, they are instructed to share their ideas with their lab partners.
Pour the water from the glass onto the plate (be careful not to spill the water onto the table). Now... Get the water back into the glass without lifting the plate!
Step 3: Some Initial Student Ideas...
Many students will want to light the lime on fire right away. I am happy to give them the matches, but I don't give them the match box to strike the matches with unless they can explain a logical reason for needing to light them. Some students will attempt to evaporate the water into the cup (my favorite). Some groups will realize the lime does not act like a sponge after squeezing out the juice. Some will come to the conclusion that they can't scoop the water off the plate without getting most of the water on the table. Every once in a while, a clever student will flip the glass upside down and see if a temperature difference might do the trick...
I do my best not to discourage any ideas. I encourage them when they are thinking in the right direction and give them hints when they are stuck. What does fire need? Are you using all of your materials? Can the glass be upside down and still hold water? We learn through failure, so it's important that they are allowed to fail. Schools often only reward students for coming up with the correct answer, but it's the process that is the most important.
Eventually, they start to piece together that they need the match on the inside of the glass. When one group starts to figure out the solution, the other groups inevitably peek and discuss what they should be doing differently.
Step 4: The Solution
Stick 4/5 matches into the lime with the heads of the matches close together. Use the remaining match to light the "lime birthday cake". Flip the glass on top of the lit matches. If the seal between the glass and the plate is strong enough, you can lift the plate right off the table. If you use real glass for this experiment, just be careful not to leave it sitting for too long because there is a risk of your glass breaking.
Here is a better explanation in writing for why the water enters the glass. Air has weight. At sea level, the air puts about 15 lbs of pressure onto every square inch (PSI) of surface area. Air is composed of several gases, including oxygen, which the fire needs in order to burn. You can demonstrate this by lighting a small candle and then placing a drinking glass over it; the candle will burn up the oxygen trapped in the glass, and then the flame will go out, since fresh oxygen from outside the glass is unable to replace it.
The drinking glass is cooler than the gasses inside the glass due to the flame of the matches. When the warm water vapor touches the cooler drinking glass, it condenses into a liquid. There's not very much liquid water visible on the surface of the glass, but there is a major change in volume: the conversion from vapor to liquid is about 1,000 times smaller. When the volume inside the drinking glass shrinks, it creates a partial vacuum—a space where there is very little gas. But a vacuum is invisible. The best way to show this vacuum to kids is to repeat the demonstration, but with water and food dye on the plate. As the partial vacuum shrinks inside the glass, the outside air (about 15 PSI), which has been pushing down on the water all along, will now push the water right under the rim of the glass, causing it to fill the glass. The water won't fill the glass completely; it reaches equilibrium. It's fun to have students analyze how to get more water to rise in the glass: a larger flame, a taller glass, colder water, frosty mug? Have fun!
I have students recreate the experiment at home for homework. They need to get a parent to sign that they were witness to the experiment. If they don't have a lime at home, they can always use something else to hold the matches. I tell them a piece of banana, apple, strawberry, potato, or any other fruit works well. I like lemons or limes because students think they might be flammable. They can also use a birthday candle and a lighter if they don't have matches.
I hope you learned something and please comment after you've had a chance to try it!