These are the tool icons. (Mac Only)

Applies a force that can move any non-wall objects. Walls with textures can be moved.
Moves all objects in the simulation.
Draws thin lines of the currently selected material and color.
Draws thick lines of the currently selected material and color.
Draws rectangles of the currently selected material and color.
Fills a region with the currently selected material and color.
Cuts rigid and elastic bodies.
Deletes objects.
Converts objects to the currently selected material and color.
If the object has a texture, only the material is changed.
Copies the material and color of the particle that is clicked on.
Clears the whole screen.
Reverts the last action.