There is a farmer who is unhappy with the milk production from his dairy farm. To rectify this, he writes to the local university asking for advice. A theoretical physicist responds to the request and visits the farm. He then takes many measurements such as the size of the cow and proceeds to do some calculations. After finishing all of this, he tells the farmer: “I have a solution, but it only works for spherical cows in a vacuum.”
The point of the joke is that in science, models are frequently used to simplify reality. Because there are infinite amounts of variables, it is impossible to predict anything unless the scenario is simplified through certain assumptions and removal of factors. For example, many physics principles make assumptions such as not accounting for air resistance. Occam’s razor states that if you shave away all the complex details, the simplest answer remains. But perhaps we oversimplify some things?