An Object

An Object
The object that measures 8 inches in length looks long and thin.It has many flat sides.But it is in the shape of a round with a 1/8-inched diameter.It also has many different colours both outside and inside.Usually it is grayish black inside and yellow outside.Besides,number either 2 or 3 is often written along the outside of it.The top of the object is made of rubber with an aluminum around it.While,the bottom of it comes to a sharp point.And the body of the object appears to be made of wood,while the inside of it is actually made of lead.Of course,it is usually used by students,artists and office workers.What is interesting is that the longer you use it the shorter and duller it gets.Can you guess what is it?