This is a variation of the classic pulling out the table cloth experiment. The pen cap can be substituted for anything small enough to fit inside of the bottle's neck. If you find it difficult, you can try using something that is shorter, such as a pebble.
An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an outside force.
In this example, the hoop is acted upon but the pen cap is not. As a result, the pen cap falls straight down into the top of the bottle.
I used a glass bottle, but it would be a good idea to use a plastic bottle in case you accidently pull the hoop down slightly.
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