Ok, maybe my bulb isn't as fully lite, but I did not know until this week that there might be a reason behind the Vault Boy's thumbs up: if mushroom cloud can be covered by your thumb, you are out of the radiation zone. 2 questions: am I the last person to know this Fallout fact and is there any real science behind it?

I would say that that has a lot to do with the size of the bomb being used. I'm no atomic expert, but the effective range spreads pretty far, if I'm not mistaken. Y'know, with radioactive fallout that potentially go hundreds or even thousands of miles.

That's a neat little bit of trivia. No idea if it was intended, but it's a nice fun idea anyways.

But no matter how big an explosion is, the farther you get away from it the smaller it will be on the horizon. That's the whole point of the using your thumb, you have to be very far away for the explosion to fit behind your thumb.

And the thumb trick is also likely more of a rough guideline for immediate survival.... Any radioactive explosion could still be harmful at your location eventually.

That's awesome, intended or not.

"Out of radiation zone" I doubt that. The wind can carry the radiation around the world.

Still it is an interesting theory.

Who the heck would stand there and check if there thumb covers it. If I saw a nuclear bomb go off I'd be running like hell to get away from it!

Edit: Interesting theory though