There is no air inside a completely filled and sealed can. But if two holes are made on the top cover of the can, air outside the can will enter it through one hole and exert atmospheric pressure on the oil from inside along with the pressure due to oil column, and it will come out of the can from the other hole.

Message: Two holes in a juice can help the flow of fluids when you are pouring a glass of juice. When you pour juice from the can, it needs to be replaced by air which flows into the can. When you have only one hole, the air trys to get in to the can as the juice tries to get out.

atmospheric (air) pressure, or barometric pressure will hold the liquid in the can, a second hole, depending on it’s location will equalize the barometric pressure allowing the liquid to flow freely.

ELI5: Why do you need to make two holes in a can for liquid to pour properly?

The seal on the nozzle of the rocket is then released and rapid expulsion of water occurs at high speeds until the propellant has been used up and the air pressure inside the rocket drops to atmospheric pressure. The expulsion of the water thus can cause the rocket to leap a considerable distance into the air.

The first man to give hope to dreams of space travel is American Robert H. Goddard, who successfully launches the world’s first liquid-fueled rocket at Auburn, Massachusetts, on March 16, 1926. The rocket traveled for 2.5 seconds at a speed of about 60 mph, reaching an altitude of 41 feet and landing 184 feet away.

: a firework typically consisting of a cylindrical case that is partly filled with combustible material and fastened to a guiding stick which may be placed in a bottle to control the direction of the rocket’s launch.

Sale and regulation. Bottle rockets are specifically illegal in many jurisdictions, even those where most other consumer fireworks are legal. They are sometimes considered to present a unique hazard, due to their ability to fly in many directions other than vertically.

Unlike the fuels for charcoal tailed rockets, whistle rocket fuel tends to be “sticky.” The high pressures used to press the fuel around the spindle cause the fuel to adhere to it. This makes the motor hard to remove from the spindle once the engine is pressed.

the stick is used to point the rocket in the desired direction of flight (usually upwards). drill a 1/8″ hole (play with the size a little for what works best for you) in the center of the bottom plug, being sure that the hole remains square.

Break the stick off a bottle rocket, light it, and observe what happens. Yeah, the stick provides stability. It does so by moving the center of pressure behind the center of gravity. The stick does provide stability in the sense that it keeps the rocket aimed in the correct direction prior to ignition.

Simple bottle rockets will fly 50 to 75 feet in the air. Eight ounce rockets will shoot approximately 150 feet into the air. Larger rockets (24” to 38”) can fly upwards of 250 feet.

Just after launch, a water rocket might reach a maximum speed of 20 metres per second, and a high-pressure rocket might reach 40 metres per second.

Tips

1. Use higher pressures.
2. Keep weight to a minimum.
3. Increase rocket volume.
4. Streamline the body of the rocket to reduce drag.
5. Use a launch tube on the launcher.
6. Use the right amount of water.
7. Use an optimum sized nozzle.
8. Use multiple stages.