My favourite analogy: Heliosphere is like water flowing from a faucet into a sink: the water represents the solar wind emanating from the Sun, and the point where it meets the sink's surface illustrates the heliosphere's boundary with the interstellar medium. Just as water changes direction and slows down upon hitting the sink, the solar wind decelerates and changes direction at the heliospheric boundary, where it interacts with the gases and particles of interstellar space.

Photo to illustrate: https://en.wikipedia.org/wiki/Heliosphere#/media/File:Helios...