What Are the Most Fascinating Idealized Entities in Physics?

[Borek]

In this thread mgb listed massless springs and friction free slopes. That made me think - what other idealized entities (not sure of that's the best selection of word, feel free to correct/edit) do we use in physics? Other things that I can think of just off the top of my head are point masses, massless strings and rods, ideally elastic strings, ideal black body, zero internal resistance batteries.

Your favorite?

 

[Stonebridge]

Containers with zero heat capacity
Pendulums that actually perform SHM (the theta = sine theta trick!)
Ideal gasses
"Thin" lenses shown as having no thickness in ray diagrams