Q:  Why was the broom late? 
A:  It over swept!

Explanation: Brooms are for sweeping; one reason that people are late is because they over sleep.  “Sleep” in the past tense is “slept” (everyday I sleep, yesterday I slept).  “Slept” rhymes with swept (swept is sweep in the past tense- everyday I sweep, yesterday I swept).  This joke is funny because the expected answer is “It over slept” not its rhyme, “It over swept.”