The idea of impossible bottles is to take things that could not possibly go through the mouth of a bottle and put them inside anyways. These tricks require dexterity, patience, and a great deal of lateral thinking.
The most common things to squeeze into a bottle are ships, unopened decks of cards, balls and other regular objects that don't normally belong in a flask. You can display these creations and let people's imagination go wild trying to figure out how these things got in there. Of course you can buy fake bottles with detachable bottoms but the real challenge is to make one yourself without cheating.
Some things are easy to squeeze in a bottle, for example you can easily deflate a tennis ball, then inflate it inside the flask, but some things seem to be impossible, hence the name: "impossible bottle".
Impossible bottle puzzle
You can purchase kits with a bottle and a wooden rod where a bolt is driven through the rod. Moreover a bolt is tightened on the bolt, stopping the rod from coming out of the glass. The challenge is to take out the stick and then put everything back together the way it was.
Watch Chris Ramsay vlogger unveiling the secret solving this puzzle:
Rubik's Cube in a bottle
Since this is a Rubik's Cube blog, let's watch Mathologer taking a break from doing fun math and explaining how to assemble two Rubik's Cubes in a glass. The video speed is increased because the procedure can take hours but you can see what's going on, especially if you adjust the playback speed to 0.25x.