This info. is slightly outdated; I stopped working at this airport about a month before Sept. 11, so I dunno how the laws have changed since then, except that knives are now a total no-no.
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I work at an airport. I'm not going to say which one, for reasons that will become obvious. We get our fair share of stuff coming through the mag (walk-through magnetic metal detector) and the X-ray (three guesses what that is); so far, we've had a pretty good record safety-wise.
We are trained to look for bombs, guns, knives, opaque items (i.e. big thick metal stuff, lots of batteries on top of each other, leaded-crystal items -- we can't see behind these, and so they need to come out so we can look past where they were with the X...) and glass bottles (can you say Molotov cocktail?)
However, in the six months that I've worked there, I've figured out how to carry an entire bomb past security without any problems.
The first thing to know is this: don't put any of the stuff I mentioned in the second paragraph in your bag or we will have to get into it, and then you're screwed. If you put a bottle in, we will have to check it; if you have a laptop (rather thick, but OK) with a lot of batteries sitting on top of it and a military beer coin in a pocket above that, you're gonna get checked.
That said, I'll move on to the meat of the essay. There are five elements to a bomb: the timer (slightly optional if you're the suicidal type), the detonator, the explosive, and the wires and battery (for the detonator and timer).
Timers can be easily constructed from an analog clock with electrically conductive metal hands: drill holes in the face and side that correspond with the required position of the hour and minute hands, respectively, and insert wires into them in such a way that the hands complete an electrical circuit when they touch the wires. The X-ray operator will not have your bag checked as long as you have the clock by itself without any other components near/attached to it.
The detonator cannot, to my knowledge, be made by hand; you must purchase a real one. Fortunately, it's easy to get past security: just get a pair of cowboy boots (or any boot/shoe that's hard to get off and has a metal shank), place the detonator inside, on the bottom, and put your foot on top of it. You _will_ set it off; allow the security people to wand you down (wand: handheld metal detector), when they get to your boots, the wand will beep. Make a rueful face and say something to the effect of "Oh, it's the shanks in my boots." They might have to squeeze around your ankles to ensure you aren't wearing a knife or gun in a boot holster, but will let you go.
The explosive is a bit more difficult; you'll need to be careful. The key is not to let anything that touches the explosive touch the bag or anything inside the bag. The best way I know of to accomplish this is to use plastic explosives:
Wearing two pairs of disposable plastic gloves over rubber gloves, mold the plastic into small (golf-sized) balls. Take off one pair of gloves. Using some type of gripper (scissors, pliers, what have you) carefully pick up the balls and put them into baby-food jars.1 Be extremely careful not to let them touch anywhere near the rim of the jar; imagine that they are made of acid and you don't want it to melt the other things in your bag. Squish them down flat so they fill the jar completely; no air spaces at all. You want to present a solid object to the X-ray operator. Take off the second pair of plastic gloves. Screw the lid of the baby-food jar on with your off hand (left if you're a righty, vice versa) then hold it with that hand and get a plastic bag with the other. Holding the bag as wide-open as you can with your good hand (right if you're righty etc.), put the jar as far inside the bag as possible without touching the bag with your off hand. Then withdraw that hand slowly and pull the bag upward with your good hand, sliding the jar to the bottom. Remove the last set of gloves, then tie the bag shut, making sure that there's no holes or cracks in it anywhere. You don't even want to squeeze it to let the air out. Put it in the middle of a wet, dirty, diaper bag, and they won't even run a random test on it because it's very easy to get a false positive with dirt/water etc.
Whew, that was exhausting! Now for an encore, the wires and battery: these should be easy. A cell phone battery ("A spare, just in case I run out...") and the wires for a power adapter should be sufficient. No problem there; even I would let somebody through with those.
Well, that about wraps it up for "Sneaking A Bomb Through the Airport Security"...
until next time, this is Bathail Klenath saying "That'll make a nice front-page blowup!"
1Why baby-food jars? Because we're only required to check glass bottles
that look like they're for alcohol i.e. wine bottles, burbon bottles; also metal hip flasks.
Another thing not to have in your bag...