Puzzle

 A container contains an unknown virus. Every day the virus doubles. On day one there is one virus. On day two there are two viruses. On day three there are four viruses and so on. On the 19th day the container becomes full of viruses. On which day is the container half full? 

Did you get tricked? The answer is 18th day. Since on 19th day its full, on 18th its half full.