i heard this joke about 10 x as i never laughed at it and someone else would tell me the same joke thinking they were a better story teller....finally i said, i don't know any french and they all laughed and explained it to me...

a couple is near the end of a pier making out. a man walks by stealthily with a bag in his hand. he goes to the end of the pier and takes a cat out of the bag and throws it in the ocean. he looks around to see if anyone is looking and throws another cat out of the bag and into the ocean.
he repeats this 3 more times. the french man looks at his lover and says, un, deux, trois, quatre, cinq. Phonetically that is un, du, twa, cat sink...

As someone learning French, I appreciate this joke. Thank you