Sheldon: Six times ten to the eighth atoms, Of a radium in a lead jar, Six times ten to the eighth atoms of radium, If the half-life of radium should happen to pass, Three times ten to the eighth atoms, Of a radium in a lead jar.
Penny: Really hope you're almost to zero.
Sheldon: No, see, that's the beauty of half-lives: it's impossible to determine when you'll arrive at zero. It's like "Wheels on the Bus" if the bus had an unknowable number of parts.