100
When illustrating magnetic or electric field lines. The lines of "force" never cross. This explains why.
What is the field is defined by a physical response (the direction of force) that the field exerts on a test object in the field. If the field lines crossed then the force applied to the test object would act in multiple directions at the same time. Recall for a magnetic field the direction is the direction that the north end of the compass points. So you can say that it is the attractive force exerted on the north end of the compass needle. For an electric field it is the direction that a positive test charge would accelerate at that point. Or we could say that it is the direction of the force on a positive test charge. (recall that the object will accelerate in the direction of the net force acting on the object)