Why does Newton's third law allow velocity change in collisions?

[joyful55]

Let me try to set N3L into a much more comprehensive example.
Try pushing your finger on a pin. The force was exerted by you on the pin but you still feel pain as if there is a force acting on your finger from the pin(because that force really exists).

 

[omin]

Thank you all for the comments. I'll keep in mind when I hit the book again.