Hypothesis: If air has is made of "stuff" it must have mass or weight. Therefore, a balloon with a lot of air in it should weigh more than an empty balloon.

Observation: When the air is released from one of the balancing balloon the balance stick should tip. Why?

Answer: This happens because there are far more air molecules trapped inside the inflated balloon and these molecules have enough weight to tip the scale.

Source: Ranger Rick's Wild About Weather.

Hypothesis: If warm air is lighter and does indeed rise, then warming the air beneath the spiral will heat the air and cause it to rise. The rising air will cause the spiral to begin turning.

Observation: In a short amount of time students should be able to see the spiral turning. If not, consider moving the spiral closer to the lamp or using a larger wattage bulb. Why does the spiral move?

Answer: As air molecules are heated they move more quickly and spread themselves thinner making a given volume of air less dense. The less dense air rises, not simply because it is warm but rather because it is lighter.

Source: More Science Projects by Rogers

Hypothesis: Water vapor in the atmosphere condenses into clouds, and then into droplets when the surrounding air temperature falls. The droplets continue to grow until they become large enough to fall as raindrops. If this is true, we should be able to duplicate the effect in the artificially created environment of the jar.

Observation: In the beginning the cloud should appear white and then turn gray. Why?

Answer: This happens because as the cold air above cools the cloud the droplets within the cloud combine and grow larger partially blocking out the light from the flashlight. When the droplets become large enough they become too heavy to float in the air and fall as rain (or roll down the side of the jar). When drops become large and fall as rain or snow it is called precipitation. Drops usually form around a dust, pollen or salt particle called a nuclei.

Source: Teaching Children About Science by Elaine Leverson

Hypothesis: Given.

Observation: Several pieces of puffed rice should jump a few centimeters from the pile to the comb. Like all static electricity experiments, this one is usually more successful on days when the humidity is relatively low.

Answer: As the comb passed through the student's hair it pick up a negative static electric charge due to friction. The puffed rice cereal had a positive charge and was light enough to be lifted when attracted by the charge of the comb.

Source: Ranger Rick's Wild About Weather.

Hypothesis: Given.

Observation: With the room darkened the children should see a small spark discharge between the two balloons.

Answer: The hypothesis that static electricity exists has been confirmed.

Source: Ranger Rick's Wild About Weather.

Thunder
in a Bag:

Hypothesis: Given.

Observation: When the bag was smacked hard with the opposite hand a loud popping sound occurred.

Answer: The air in the paper bag made a loud bang when it was forced to expand out of the bag rapidly due to compression of the bag. When lightning occurs it super heats the air around it. The super heating causes the air to expand and move very rapidly from one location to another making the sound of thunder.

Source: The Weather Dude website.