Clouds in a bottle

This simple classic forms a little cloud in a bottle. You will need a large glass jar with a wide mouth, such as an industrial size pickle jar; a sheet of rubber (a balloon that has been slit open); a rubber band large enough to fit over the mouth of the jar; some chalk dust (or talcum powder); and some cold water. Wash out the jar and put about an inch (25mm) of water in the bottom. Cover the mouth of the jar with the

piece of rubber and cover with a book to hold it in place. After 10-15 minutes, remove the book and rubber from the jar. Drop in a spoonful of chalk dust or talcum and quickly replace the rubber over the mouth. Wrap the rubber band tightly around the rim to keep the rubber sheet firmly in place over the mouth. Push down on the rubber with your fist until it is depressed a little way into the jar. The air is compressed and warmed, so it is able to hold more water vapor. After 15 seconds or so, quickly remove your fist. The air will cool, and will not be able to hold as much water vapor. This excess vapor condenses around the chalk dust, forming a cloud inside the jar.
You can do the same thing with a jar or large bottle and a cork The cork needs to make a good seal, so give it a good soaking if it is dry. Put a little cold water in the jar. Cork the jar, and let it stand for 15 minutes or so. Remove the cork, quickly pour in a little talcum powder or chalk dust, and re-cork. Push the cork farther into the jar. This increases the air pressure, warming the air inside. After 15 seconds or so, pull the cork a little bit of the way out (but not all the way - you need to keep the seal). The pressure drops, the air cools, and the excess water vapor condenses on the tiny particles of powder in the air. 

Sound rings 

Under certain conditions, short bursts of sound will set up peculiar vibrations in the air known as "sound rings". These rings carry a lot of power with them. Some have been produced that will knock over relatively large objects at several meters away!

You can create small versions of these that will blow out a candle. Use a round cardboard box, such as an empty oatmeal box. Cut a round hole the size of a penny (U.S. - 37mm) exactly in the center of the lid. The hole should be as perfectly round and smooth as you can get it. Tape the lid tightly on the box.

Point the hole in the box top at a candle flame three feet (one meter) away. Sharply tap the bottom of the box. Sound rings should form which should be strong enough to put out the candle. By trial and error you can determine how far the box can be from the candle and still put it out. You may be able to get 6 feet (2 meters) or so away.
You can make the rings visible by adding smoke. Do this outdoors, or at least someplace well ventilated. Remove the lid, light a damp paper towel so it just smolders, hold the paper towel in the box to fill it with smoke, and seal the lid back on. Make sure you have a safe place to put the smoldering paper towel when you are done with it! When you tap on the box, you should be able to see the smoke carried by the sound rings. 
You can also make a sound ring generator with a plastic soda bottle and a toy balloon. Cut off the bottom of the empty bottle as evenly as possible. Sand it smooth, or heat it so that it can be smoothed. Cut the balloon so that you have a piece of rubber large enough to cover the hole in the bottom of the bottle. Tape it into place. Duct tape works well for this. The balloon should be fairly taught. Grab the balloon in the middle and pop it. This will give a fairly strong little burst.
Ryan Hersh writes to add the following design suggestions:

• Both ends of a tin can are removed. A toy balloon is attached to one end and a piece of cardboard with a hole at the other.
• A larger one can be made from a plastic trash can with a flat lid.
• 10-gallon buckets that detergent/paint/some bulk foods come in also work well.