Modern security screening in airports are highly automated systems, with only a limited number of humans interacting with it at key points (maybe that's why it breaks down so often?).
Computers control sorting, weighing, screening and transport almost all the way out to the plane.
Security, however, can't always be trusted to machines, who can read patterns well, but has, at times no more sense than a cockroach in a snowstorm.
That's why your bag, if you're just having that kind of day, will sometimes be picked out for closer scrutiny by human eyes.
  When your bag comes floating down into the baggage handling system.
it is first sent through an x-ray machine that will determine whether it contains a pattern of items and a density analysis that consist with a weapon of some kind.
If the machine thinks there might be some kind of fishy business going on, the bag will then get sent through to another x-ray machine, where the image it produces will be reviewed by a human.
In more than nine out of ten cases, the human will determine that there is no monkey business afoot, and your bag will go on its merry way to the plane, and back to you without anyone ever even thinking of opening it.
  There are, however, the times when the security officer can not see what's in your bag, and that means it has to be rummaged through.
There's nothing fishy about it, an believe it - the officers have seen everything before.
Twice.
So even if you had something in your bag that you feel bad about some stranger seeing, never fret.
They won't tell, since there's nothing new under the sun in the baggage handling system.
  But anyway, with the x-ray systems growing ever more advanced - 3D images are getting common - how can you make sure your bag isn't selected for search? The answer is in the organization.
1; Pick a large enough bag or suitcase.
It should be so large that you don't have to cram everything down and then sit on the thing to get it to latch shut.
  2: Organize things correctly.
If you have to bring electrical equipment, put them spread out through your bag.
Wires should be arranged in a coil, and with the plug by itself - imagine taking a picture of the item where it is in your bag.
You should be able to discern every part of it clearly.
  Cheese, marzipan, vacuum packed coffee or dry goods and other dense food items or organical items will be analyzed as having the same density as explosives.
Consider not bringing such things unless you absolutely must.
  3: Separate items.
Put your clothes, liquids (i.
e.
toiletries and such) and objects containing metal (electrical equipment, computers, phones, etc) in separate areas of the bag, preferably either tie or tape some separating cardboard between them if there's a chance the things might bounce around and mix with each other.
  4: Remember to tag you luggage with your name clearly printed on a laminated card securely affixed to your bag(s).
Many countries (especially in Europe) has legislation that demands the owner is presented when searches are conducted, and you can be sure that your plane won't lift off before they can find you.
Either that, or your bags might be left behind and dealt with by police.
(And the police aren't polite baggage handlers...
) If you're going on a trip and you're planning on bringing carry-on luggage, take a look around the internet - there are many reputable sites regarding this, including the TSA.
gov site - their "How to pack" page is pretty.
Lastly, you can never be sure that your bags won't be sure, but these tips will certainly bring the chances up in your favor!