This one is almost impossible to describe... One apple asks the other their apple type: "Birs?" (This type is called "quince " in English). But "birs" is similar to "birsz", meaning "Do you like me?" said with a lisp. And the answer is "More than that, I love you", said with a lisp. In short the joke is that the second apple thinks that the first is asking if they like them or not, answering "yesh", signaling that they have a lisp.
- Dad, if we know where something is, is it lost? - No son. If we know where it is, it's not lost. - Good. Then I didn't lose the car keys in the river.