Let's try one...
There are this Jar standing on a table in a lab. In this jar there is a (what's the english word?) bacterium, whitch multiplies. In fact, it doubles itself every minute.
So, the doctors leaves the jar unguarded (at 1 pm, just in order to get a certain time), and the bacteriums starts to multiply. After half an hour, the jar's half full. When is it full?
Übereil
<span class='standouttext'>Spoiler : </span><span class='spoiler'> At 31 minutes. </span>
