What will we tell our children??

What will we tell our children when they get older to know what happened on April 15, 2013??

When my 20 month old is able to go out by himself, what will I tell him??

When he is a teenager and able to go out with his friends, how will I keep him safe?? I know this is a question of all parents, from all generations... But will he know when he sees a strange "bag" or object in his views that it could be a bomb?? When he wants to see a typical and traditional American sports event, will he be safe? Or even a new release movie, will he be safe in the theater?
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How will I prepare him to go out into this world and be safe??

Yes, there are more good people in this world than bad... Yes, the chances of something happening to him are so slim....

But even knowing that, how will I know MY son is safe??