I don't quite understand these cold jokes.

There is a student who doesn't study hard and draws circles when he encounters words that are difficult to write and words that he can't write. On this day, his uncle was ill in hospital. The student wrote a letter to his uncle. He was going to say, "Dear old uncle, I heard that you were ill. You should take good care of yourself and never get out of bed casually. " The meaning is quite good, but because I can't write a few words, I drew a circle to show it, which made a big joke. His letter reads: "Dear Lao, I heard that you have a ○. You should take good care of it and don't get off at will." The letter was sent to the ward, and his uncle was inconvenient to move, so he asked his aunt to read it to himself. When her aunt saw several circles on the letter, she asked, "How do you pronounce circles?" His uncle said, "Say' egg' when you meet a circle." So menstruation began to read: "Dear old egg, I heard that you have eggs. You should take good care of them and don't leave them anywhere. " After reading the letter, his uncle was almost angry. Hope to adopt. I like jokes. Thank you.