The chalk particles embed themselves into the small pores on the surface.
Although a chalkboard seems smooth to the touch, it is quite rough at the microscopic level, with <em>pores</em> that reach below the surface.
When you drag chalk across the board, friction causes small particles of chalk to rub off onto the surface.
If you leave the markings for a long time, some of the chalk particles will work their way into the pores.
A brush will remove the surface particles, but <em>it will not be able to get at the particles in the pores</em>.
If we are talking about moles then the answer to that is 0.22
It's because of the gravitational forces
It was a spring morning of April 16, 1947, was a morning that many citizens of Texas City thought was the end of the world. The French Liberty ship, SS Grandcamp, bearing a cargo of ammonium nitrate fertilizer (over 2300 tons) destined for war-torn Europe, caught fire in the Texas City harbor. The bright ... A little after 9.00 a.m., the Texas City Disaster (as it is popularly known) occurred as the ship Grandcamp exploded.