Air Pressure experiments: Self inflating bag & Fountain in a bottle

Air power! Use 2 plastic bottles to magically inflate a bag sealed in a bottle, or create an endless fountain inside a bottle! Use these experiments in a magic science show or to wow friends and family members, while learning about the effects of air pressure!

Bottle Connector

To start off you will need to create a bottle connector. This is simply 2 bottle caps connected together with 2 holes punctured in them. The holes must be small enough to tightly fit straws in them

Things you need:

Bottle connector: 2 Bottle caps 

Self inflating bag: small plastic bag, rubber band, straw, bottle, large container, water

Fountain in a bottle: 2 straws*, pin, 2 bottles, water

*For the straws I recommend using stiff kind so it does not bend easily when fitting into the connector. I got my straws from a bubble tea shop, you need 2 of them to fit in the bottle cap so make sure they are not too big!

Step 1: Create the bottle connector

As you need the connector to be water tight, I suggest you first glue the 2 bottle caps together, hot glue gun or epoxy will work. Once it's dried, wrap them together with water proof tape. In the video I used electrical tape, which is a bad idea so off camera I wrapped duck tape first.

Connect 2 bottle caps together

Step 2: Poke 2 holes into the caps.

You will need to create 2 holes between the caps, and they must be small enough to fit the straws. Make sure the holes are a distance apart so when widening the holes they will not collide. I highly recommend using a drill, the method I used in the video can risk you stabbing your hand so don't be like me and make sure your hand is not behind the bottle cap!

This is dangerous and can risk your other hand being stabbed!

Create a small hole using a push pin to mark out where I want to create the hole.

After creating the starting holes I proceed to widen it using a hand drill and scissors until a straw can fit in tightly. While doing this you may risk making the hole too wide, in that case you can use plasticine or blu tak to secure the straw in place, or wrap tape around the straw so its thickness increase and you can fit it in again.

Widen the hole until the straw can fit in tightly.

You are now left with a bottle connector with 2 holes. You can clean the holes with a small file.

Experiment 1: Self inflating bag

First is to create the gimmick. Take a plastic bag and wrap its mouth on one side of a straw using a rubber band.

Slot the other end of the straw into the hole in the bottle connector.

Fill the water bottle 1/4 full of water. Flatten the plastic bag and insert the bag side into the bottle and screw the bottle cap on.

Lastly, turn the bottle upside down into the container and let the water flow out. The bag inflates!

Explanation

As water flows out of the bottle, surrounding air pressure outside the bottle becomes greater than the pressure inside the bottle. This causes air to be sucked into the straw, inflating the bag.

Experiment 2: Fountain in a bottle

For this next experiment you will need 2 straws and 2 bottles. First insert the straws at opposite ends. At the long side of the straw, use a marker and mark where the straw touches the connector.

Remove the straws. Using a pushpin, poke holes around the straw above the marked line. Do not poke holes too high up the straw! 

Insert the straw back into the bottle connector, ensure the holes are close to the connector base.

Next, fill a bottle with water, you can add food colouring for the fountain. Attach one end of the connector the it and the top with an empty bottle. Your fountain and a bottle is ready!

To start the fountain, simply turn the bottle upside down!

Explanation

Similar to the self inflating bag, as water flows into the bottom bottle through the lower straw, air is forced up the tall straw. As there's holes in the straw, water also flows into the taller straw, which causes the air moving upwards to push the water up, creating a fountain!