Well, you check memory very briefly, make a quick scan, and it's awfully easy to say to yourself, Ring, Rang, Rung, and very hard to say to yourself, Pare, Park: they come more slowly.
你又开始迅速回忆， 很容易想到 Ring，Rang，Rung， 很难回忆出，Pare, Park： 至少要慢很多。