Your mom is so small that she can fit in the luggage.
Mom in Luggage
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Explanation
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Explain Bear
Okay, so basically, the joke is saying the person's mom is super tiny, like so small she could curl up and fit inside a suitcase. It's funny because moms are usually regular-sized humans, not travel-sized accessories. You probably have the same joke told to you all the time because you are also pretty small.