You've probably heard someone being characterised as having "the attention span of a goldfish", meaning that he or she can only focus on something for a very short time. If you search for the actual attention span of a goldfish, you'll even come across a recent study that found that the average human attention span is now less than that of a goldfish (human: 8.25 seconds, goldfish: 9 seconds).
So what does that mean for presentations?