Thinking about weird stuff - What is your favorite thought experiment?

Here’s an example: You drop a teaspoonful of ‘white dwarf matter’ or ‘neutron matter’ onto your toe. What would happen?

A thought experiment allows you to do things that normally wouldn’t work. Like how to get neutron matter onto your teaspoon. Here’s Wikipedia’s definition:

A thought experiment (a calque or English loanword based on the German/Latin compound Gedankenexperiment), is a proposal for an experiment that would test or illuminate a hypothesis or theory. Given the structure of the proposed experiment, it may or may not be possible to actually perform the experiment and, in the case that it is possible for the experiment to be performed, there may be no intention of any kind to actually perform the experiment in question.

The common goal of a thought experiment is to explore the potential consequences of the principle in question. Famous examples of thought experiments include Schrödinger’s cat, illustrating quantum indeterminacy through the manipulation of a perfectly sealed environment and a single radioactive atom, and Maxwell’s demon, in which a supernatural being is instructed to attempt to violate the second law of thermodynamics.

Care to share one of your thought experiments?