Couples want to buy knives, but they buy food on the way to pay the bill.

The couple were going to buy knives, but on the way, the husband suddenly said to his wife, "Let's buy something to eat, or what to eat tonight?" The wife thought about it and found it reasonable, so they went to the vegetable market and bought many dishes, including potatoes, carrots, beef and so on.

When they were about to leave with all the dishes, the wife suddenly remembered, "Aren't we here to buy knives?" My husband paused for a moment, then smiled and said, "Nothing, we'll eat vegetables tonight and buy knives tomorrow."

So the joke means that when you do one thing, sometimes you will be disturbed by other things, leading to forgetting what you originally wanted to do.