排列组合的问题

steffifu

OG数学里面的一个例子，硬是没想清楚，求大神解释一下：

each time a coin is flipped, there are two possible outcomes, heads and tails. If an experiment consists of 8 consecutive coin flips, then the experiment has 28 possible outcomes, where each of these outcomes is a list of heads and tails in some order.

这个28是怎么算出来的?
谢谢。