/sci/ - Science & Math

I still haven't fully grasped Newton's third law. Yeah, I'm still stuck on that horse and cart problem.
Look, I press down at a key on the keyboard and, as an effect, the force applied is replied in equal magnitude but opposite direction. If I apply a varying magnitude of force to the key as such:
1/20N, it responds with -1/20N
1/10N, irw -1/10N
1N, irw -1N
KN, irw -KN
The minus sign to indicate inverse direction and K is any magnitude to generalize its consequent, -K, inversion independently of the K force apply. Obviously these forces don't cancel out, I wouldn't be typing if they did, but please explain to me where I'm wrong here. Someone once answered me that these interactions does happen, in all magnitudes, but still it doesn't mean that it should cancel each other out. So now I'm left clueless.