We might understand Roadrunner's confusion by considering Newton's third law explicitly. According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. A force may be defined here as a push or pull upon an object which results from its interaction with another object. Newton stated his third law of motion exactly as:
"To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts."

To restate it, "For every action, there is an equal (in size) and opposite (in direction) reaction."
To nail this concept down, consider classroom example 1:
Q: An unfortunate bug strikes the windshield of a bus moving down the road. Quite obviously, this is a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the bus windshield?
A: Each force is the same size. For every action, there is an equal ... (equal!). The fact that the bug splatters only means that with its smaller mass and strength, it is less able to withstand the force resulting from the interaction.
Now consider example 2:
Q: A speeding plane strikes the wall of a ("motionless") Tower. Obviously this contact between objects is a case of Newton's third law of motion. An aluminum plane hits the steel building and the building hits the plane. Which of the two forces is greater: the force on the plane or the force on the building?
A: Each force is the same size. For every action, there is an equal and opposite reaction. The fact that the steel beams, spandrel belts, and steelreinforced concrete floors of 56 stories shatter, fragment and form an airplaneshaped gash...OOPS...you're kidding! This correction just in: the fact that an aluminum plane crumples, shatters and leaves no "airplaneshaped silhouette of passage" in a collision against massive quantities of structural steel only means that with its lower mass, density and strength relative to the building, the plane is far less able to withstand the equal force exerted on both bodies.
We can check our understanding with a few calculations. Each WTC Tower weighed approximately 500,000 tons. As a first approximation, if a plane hit the upper five floors, these floors would weigh approximately 22,727 tons (5 floors divided by 110 floors = 4.5 percent of 500,000 tons). A Boeing 767 would weigh approximately 140 tons flying as described by government and media. The mass of such a plane would be 0.6 percent of the mass of five floors in a Tower (140/22,757). Therefore, the aluminum plane would be less than one percent of the mass of the section of the steel/concrete building it allegedly hit. The plane's material density and strength (resistance to forces like bending, etc.) would also be vastly inferior to those of a Tower.
Conclusion: byebye airplane.
Critics may object that the steel beams in upper stories were thinner than those in lower floors (less building mass to support with nothing else lighter on upper stories), so generously shrink the estimated weight of the upper five stories by onethird, yielding 15,152 tons. The mass of a 767 would be 0.9 percent of the mass of five floors it theoretically hit. To restate it: the mass of the building section allegedly hit by the hollow aluminum tube with engines and wings and no crash rating is 100 times that of the plane.
Result: byebye airplane.
And what if the mass of the plane were even smaller?
Same answer: byebye airplane.
What if a plane flew faster into the steel tower? Impact force increases but remains equal for plane and tower at any speed. The tower is immensely stronger and harder than the plane.
Upshot: byebye airplane.

Figure 8. Threepound bird goes mano a mano with aluminum plane and does heavy damage.
source 

Figure 9. Airliner damage by bird impacts.
source 
What about the belief that fragile objects with sufficient speed penetrate massive, hard objects, e.g., tornadoes allegedly driving straws into trees? According to a government source, this can happen because "intense winds can bend a tree or other objects, creating cracks in which debris (e.g., hay straw) becomes lodged before the tree straightens and the crack tightens shut again."
