Here's why:

* Magnetic Poles: Bar magnets have two poles, a north pole and a south pole. The magnetic force is strongest at these poles.

* Magnetic Field Lines: Magnetic field lines represent the direction and strength of the magnetic force. They always form closed loops, originating from the north pole and entering the south pole.

* Density of Lines: The closer together the magnetic field lines are, the stronger the magnetic force. At the poles, the lines are packed very closely together, indicating a high concentration of magnetic force.

So, if you were to sprinkle iron filings around a bar magnet, you'd see a dense cluster of filings at the poles, demonstrating the strongest magnetic force in those areas.