Question

If we drop a pen then it fall in the ground of earth. But why the earth don't fall into pen ???!!!!!

Answer 1

earth has it's own gravitational force but the pen doesn't

Answer 2

the Earth technically does move toward the pencil, but the amount is so small as to be unmeasurable and essentially ignorable. This is because the mass of the Earth greatly outweighs the mass of a mere pencil (and as Guillem Usach points out, the formal answer is that this is a question of reference frame).

But there is another way of looking at this. Yes, the pencil is small and the Earth is large, but perhaps more important, gravity is an incredibly weak force. Why doesn't the pencil (or the Boeing 787 for that matter) affect the Earth? Why do objects outside of our solar system have no perceptible effect on Earth? Because all this "other stuff" in the universe simply cannot conjure up enough gravity to move the Earth, not only because the Earth is heavy, but because gravity is pitifully weak.