You glance at a clock and see 3:00. It stopped yesterday, but by sheer luck it is 3:00 again. Did you know the time?
Your belief is true: it really is three. You also had a reasonable-looking reason to believe it; the clock has always worked before, as far as you knew. Still, the truth arrived by coincidence. If you had looked a few minutes earlier or later, the very same glance would have misled you.
Cases like this challenge the idea that knowledge is simply a true belief supported by good reasons. The belief seems justified and correct, yet luck appears to have done too much of the work. Philosophers disagree about what else knowledge requires: perhaps a reliable way of arriving at the truth, perhaps a different link between evidence and fact. The puzzle reveals a gap; it does not close it.