Once upon a time, there lives a young man who likes to read aloud, but he never thoroughly understands his literature's contents, or he only catches part of the importance of what he is reading, or maybe he does not grasp the importance at all. Yet he considered himself knowledgable. One day he went to buy some fruit. On the market the old man selling pears told him that raw pears are good for one's teeth but bad for one's spleen. Another old man selling dates told him raw dates are good for one's spleen but bad for one's teeth. Hearing those words the self-important man said he knew a way to enjoy the nutritional benefits from both, while avoiding the negative side effects. "I could chew pears, but don't swallow them and swallow dates without chewing them." he said. "By doing this, both of my teeth and my spleen will not be hurt." He did what he said and he was choked by a date.
This joke gave rise to the idiom, which refers to lapping up information without digesting it, or reading without comprehension.