You cannot do so, because it takes something with a rougher surface than paper to wear off the point of a slate-pencil. A slate is used to write on with slate-pencils, because slate wears off the end of the pencil easily, and also because you can rub out the writing on a slate with water. Lead-pencils are used for writing on paper, but you must have a rough surface on the paper to write on even with a lead-pencil. Some kinds of papers have such a smooth surface that you cannot write on them with a lead-pencil. How Does a Pen Write? Writing with a pen, however, is quite different from writing with any kind of pencil, because in writing with ink we do not wear off the end of the pen, but have the ink flow from the pen. For this purpose we must have a surface that will absorb the ink from the pen, and draw the ink down off the pen and make it flow. A slate has no power of absorption and therefore cannot draw the ink. A piece of blotting paper is the best kind of paper for absorbing ink, but it is too much so for writing purposes. For writing with ink we need a comparatively hard surfaced paper that has absorbent qualities, but not too absorbent. How Does a Blotter Take Up the Ink of a Blot? It is because the blotter has a very excellent ability to absorb some liquids. The thinner the liquid the more easily the blotter will absorb it. Ink is thin--being mostly water--the blotter is of a loose texture and has a rough surface. This gives the blotter the ability to pick up the ink, just as a sponge would do. A sponge has what is called the power of capillary attraction and so has the blotter. Where Does Chalk Come From? Deposits of chalk are found on some shores of the sea. A piece of chalk such as the teacher uses to illustrate something on the blackboard at school consists of the remains of thousands of tiny creatures that at one time lived in the sea. All of their bodies excepting the chalk--called carbonate of lime in scientific language--has disappeared and the chalk that was left was piled up where it fell at the bottom of the ocean, each particle pressing against the other with the water pressing over it all until it became almost solid. It took thousands of years to make these chalk deposits of the thickness in which they are found. Later on, through changes in the earth’s surface, the mountain of chalk was raised until it stood out of the water and thus became accessible to man and school teachers. How Did Men Learn to Talk?