Help
= move or select objects (hold shift to move individual atoms that are part of molecules)
= add a particle of type A by clicking with the mouse

= add a particle of type B by clicking with the mouse

= add a particle of type C by clicking with the mouse
= add a particle of type D by clicking with the mouse
= add an obstacle by dragging with the mouse
= duplicate an object
= add text
= delete objects by dragging the mouse over them
= click a particle to show its trajectory
= highlight particles by dragging the mouse over them
= count particles by dragging the mouse over them
= click to show the velocity vectors