In 1935, Erwin Schrödinger, a famous quantum physicist, devised a thought experiment in an attempt to explain the Copenhagen interpretation of quantum physics (which posits that the state of every particle can be described by a wave function, and that the process of calculating its position determines it). It goes as follows:
A cat is placed in a box with a sealed vial of poison that is set to release when a radioactive material has decayed. As the time taken for the decay varies (roughly 50:50 chance), it is unknown whether the vial has broken after an hour if the box is closed. This also means that it is unknown whether the cat is alive or dead. To solidify this variable reality, one must open the box, whence the cat is determined as either alive or dead.
This experiment may be hard to understand for a non-physicist, but it is still a fascinating thought experiment. This is because there are many times in life where one cannot know the outcome of something unless action is taken. Ergo, if you want a state of uncertainty, do nothing; if you want a set answer, take action. This is a particularly useful answer to someone questioning whether they should start a relationship with someone and are unsure of the outcome.