Probability - How does it work?!

But surely in the case of only 100 tosses (fnarr) 100 heads in a row would mean a coin that'd been tampered with?

I think it's in Taleb's book (Black Swan) the anecdote about a Maths type insisting that a coin that's landed 99 times on heads still has a 50% chance landing on tails the next throw, while some more streetwise trader suggests that he would place his mortgage on the coin landing heads again (because it's so obviously been tampered with).