There are security checks that can prevent even that, but cavity searches wouldn't be enough. So let's not discuss them. Suffice it to say that hardly anyone would get on an airliner any more. This might be a success in TSA's eyes since they've been aiming toward that for their whole existence. But I don't call it a win.
Could we just say the tradeoff isn't worth it and let everyone bring their shampoo bottles on board? No. What makes a war different from struggling against nature is that we're facing an animate enemy who reacts. If we skimp on maintaining the Missouri River levees Mother Nature won't abandon tornadoes and put all her efforts into thunderstorms. But Islamofascist terrorists will switch their efforts to whatever security hole gives them the best shot at killing the most people.
The problem is the premise of America's entire airline security effort:
It doesn't matter if terrorists get on the plane as long as we take their weapons away.Which is futile. There are too many possible weapons, including the human body with good training. There are too many objects being taken on planes to thoroughly check them all. There's too many inventions being made for a passive defense to recognize everything dangerous. Trying to make it work leads to the reductio ad absurdum patgund described while destroying one of the great benefits of modern civilization, the freedom of personal movement.
There's a better way. It's even being used. Israel doesn't rely on minimum wage x-ray monitors to keep El Al planes safe. It looks carefully at the people coming on board and questions anyone who looks suspicious. Ethic profiling? Religious profiling? Of course. Let's get real. The typical suicide terrorist has a lot of common features. Muslim male, sometimes a recent convert, regular attendee at a mosque with sermons advocating the death of Jews/Americans/etc., often spent several years on welfare. Yes, there are terrorists who don't meet that profile. But starting with them is much more effective than picking random subjects for close scrutiny. And it's light-years better than harassing Swedish grandmothers to protect yourself from ACLU and CAIR lawsuits.
Screening isn't the only line of defense. There's also armed air marshals. We have some now, though they aren't as "undercover" as they could be for maximum effectiveness. Arming pilots has been proposed but TSA has thrown up roadblocks for any pilot actually wanting to defend his passengers. The best way to have someone in place to resist hijackers would be the militia. There's a lot of frequent fliers out there with military experience, or willing to go through some training to become an auxiliary air marshal. Don't pay them, just authorize them to carry weapons when they're on the plane anyway. That's a cover no government agency can duplicate.
While we're screening people we can move some into the fast lane. Frequent fliers could be pre-screened to by-pass airport security (with some very stringent ID checks). That leaves more resources for the unknowns. By then a lot fewer terrorists can get through. With more armed passengers to resist the odds of a successful hijacking are much lower than in our current system. All we need to do is:
1. Face up to who's actually trying to kill us.
2. Empower citizens to fight back.
I'm not sure which of those is harder for our government.