ghostrider said:So, if a terrorist can use a knife to take over an airplane on which I am a passenger, then I dont need a knife?
Note that "you" was used in both parts of the sentence, but you replaced one of them with a reference to another person. Let me rephrase their statement for you:
If A PERSON can hijack an airplane with a knife, then THAT SAME PERSON does not necessarily need that exact tool to do that job. Due to the nature of the job, if it can be done with a knife it can be done a bunch of other ways to. Therefore, a knife is not needed and a ban on knives will not solve the problem.
Note that these are "if" statements, so there is no solid assertion that it CAN be done as easily as it is said, but IF it can then a knife surely isn't the only magic tool that can do the job.