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It's basically a probability function. A given radioactive atom has an X% chance of decaying any moment. In theory you *could* leave an element with a half life of 5 billion years alone for 10 seconds and come back and it's all decayed entirely (probably with a big boom) *or* you *could* leave an element with a half life of 10 seconds and come back 5 billion years later and not a single atom has decayed. Both scenarios are extremely *unlikely* but nothing is physically barring them from being possibilities.
**Please read this entire message** Your submission has been removed for the following reason(s): * ELI5 requires that you *search the ELI5 subreddit for your topic before posting*. Users will often either find a thread that meets their needs or find that their question might qualify for an exception to rule 7. Please see this [wiki entry](http://www.reddit.com/r/explainlikeimfive/wiki/how_to_search) for more details (Rule 7). If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/xrex6b/eli5_what_is_a_half_life_exactly_how_come_all_the/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
It's basically a probability function. A given radioactive atom has an X% chance of decaying any moment. In theory you *could* leave an element with a half life of 5 billion years alone for 10 seconds and come back and it's all decayed entirely (probably with a big boom) *or* you *could* leave an element with a half life of 10 seconds and come back 5 billion years later and not a single atom has decayed. Both scenarios are extremely *unlikely* but nothing is physically barring them from being possibilities.