This is a typical type of joke (called a spoonerism), where two things are compared and the first letters of two words are changed around. A tired teacher marks badly (meaning they correct the students' work badly), but an angry dog barks madly (makes a lot of noise).
這是英語中稱作spoonerism的笑話，意思是兩個相近而第一個字母互換的詞。Marks badly是老師改功課改得差，而barks madly就是一直在吠製造噪音。■漫畫&說明︰英國文化協會
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