Best of PSE 2016 - Best baby-steps answer

Question

What was, in your opinion, the best baby-steps answer in 2016?

A baby-steps answer starts from simple principles and finishes with a complex and rich result.

Vote either by posting a link to your favorite baby-steps answer, with a brief description of why it's your favorite, or by upvoting an existing post. Voting lasts two weeks, i.e. until Jan 29 2017.

This is part of the Best-of PSE 2016. If you are interested in providing a prize bounty for the winner (mostly highly upvoted answer of this meta post), please comment.

Answer 1

Answer to "What do we mean with magnetic monopole and dipole?"

The answer assumes that the OP is new to magnetostatics but is good at electrostatics.

The answer starts by drawing the fundamental analogies between the electrostatics and magnetostatics. The answer then slowly explores more analogies through simple derivations and ultimately ends up answering the all of the OP's questions. On its way, it explains important concepts using simple ideas, for example, it explains why magnetic field lines are always closed or why a zero exists on the right hand side of the Gauss Law.