That's a lo-o-o-o-ng time!

Question - why, when talking about radio-active material decaying, do we refer to "half-life"?  Why half??  Well, do you remember when we were discussing Zeno's paradoxes?  It's a bit like that - suppose you have 100g of uranium 235, which has a half-life of 700 million years.  This means that in 700 million years time there will only be 50g of it still active.  Another 700 million years after that, it will be down to 25g and so on.  And of course Zeno tells us that halving the amount like this every 700 million years means that we shall never reach zero - there is no such thing in decay terms as a "full-life", or more to the point, the time it will take our uranium 235 to completely decay is infinite.