We will make a barometer to measure changes air pressure. Air pressure is caused by the molecules of air piled up on top of each other and pushing down. We do not normally notice these changes, but may have done if we have been in an airplane. The air pressure is quite a bit lower up high, so much so that airplane cabins need to be artificially pressurized so that we can breathe up there.
How to make a bottle barometer, which can show changes in air pressure:
Add the water to the empty drink bottle. Optional: add several drops food dye.
Insert the tubing into the bottle until one end rests on the bottom of the bottle and the other end hangs out of the bottle.
Suck on the tubing end outside of the bottle, until the water almost reaches your mouth.
Take your mouth off the end of the tubing and immediately plug the end with some clay/playdough. If the water level inside the tubing drops out of sight before plugging with clay, retry until the water line is high enough up the tubing to be clearly in view.
Watch the level in the tube for a little while to make sure the modelling clay makes an airtight seal on the end of the tube.
Add a piece of tape to the outside of the tube with a line on it, or on the wall next to where it is taped, to show the initial level of the water.
The level in the tubing will rise up and down as the atmospheric air pressure changes over several days:
Air pushes down on the water surface inside the bottle, which holds the water up in the tube.
If the atmospheric air pressure rises (usually associated with clear weather), the air pushes down more on the water surface inside the bottle, which pushes the water level further along the tube.
If the air pressure drops (usually associated with rainy weather), air pushes down less on the water surface inside the bottle and the water level in the tube will drop.
To immediately see how the water level in the tube changes, use your breath to change the air pressure inside the bottle:
Cut a straw in half and wrap more modelling clay around it, sealing to make sure that there are no air gaps.
Insert the straw into the barometer bottle mouth, and use the clay to seal over the top of the bottle and around the tubing.
Blow into the straw. If their is no air leakage, you will make the air pressure in the bottle rise, which will push the liquid up the tubing.
It takes a lot of puff to move the water line only a little. This gives a sense of the pressure changes that occur in our atmosphere regularly, but we do not notice so much (unless we have arthritis which can cause sensitivity to pressure changes in the joints).