This is because the objects are unstable, or are sensitive to small numerical errors. By using an adaptive solver, the simulation can be made more accurate only when necessary. This results in a more stable system which runs at a reasonable speed. There are four walls. In case you haven't guessed by now, they are the top, left, right and bottom walls.
Walls are only one-way.
An object moving from the screen toward a wall will bounce off the wall. But an object moving from off screen toward the screen will pass through the walls. Note that for large numbers of masses, this can get pretty slow. By pushing the Save State button, the current system state is saved. If you break it, you can Restore State any number of times you like.