Some scientific concepts are brilliant in their simplicity, but others require a little mental gymnastics to really understand. Today we want to know about the scientific tenets that you find trickiest to grasp.

Whether it's an idea from theoretical physics or just a question of a seemingly impossibly large scale, tell us about your pick in the comments — along with anything (perhaps a picture, a video, a visualization, or even a GIF) that finally made you say "A-ha!"

Image: Artist's concept of a black hole / NASA/JPL-Caltech.