Cops visit a guy who was stealing railroad tracks and selling them as (s)crap iron. - Where did you get these tracks from? - I don't know, the kids are playing around, they must have brought them. - How could kids have carried this - it's 300 kilos each? - What does a kid know what's 300 kilos, they just pick and take away...

Now why is this a programming joke?

Anyone who did something big, and was not told in advance how big and hard it would be... has had their 300 kilograms. Or sometimes you even suspect it may be that hard, but you actually don't want to know. You just do it, and get scared later.