To tread means to step on something. Corn has two meanings - it is a vegetable, but it is also a piece of hard skin on your foot that is painful. If you stepped on either type, a farmer would be upset！
如何傷害一個農夫？既可以踏（tread）他種的粟米（corn），也可以踩他的足蹠疣（corn的另一種解法，也就是俗語說的「雞眼」）。■漫畫&說明︰英國文化協會
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