A stupid joke

When I was a child, I made a joke for my grandmother.(I think in this case "prank" would be a better description instead of "joke". Usually jokes are spoken and are supposed to make people laugh and pranks are typically for amusement)
My grandparents lived and live near me so one day, I don’t remember the reason, I thought of a particular joke for my grandmother.
I think I was 10 years old. It was morning. I took a sheet of paper and I wrote a letter to my grandmother. In the letter, I wrote that she had no more money in the bank and I wrote that the letter had been personally written because it was very important . To make it look more real, I used the envelope of the bank (it was arrived a few days ago, and in the envelope there was our name and address; my parents opened the envelope without destroying it, so it was simple to use it again). I used formal verbs (how I was so clever!) and I wrote in a different way because she couldn't understand my child writing. It was so perfect! I felt so cool!
I put the letter in our mailbox. When I saw my grandmother take the letter, my heart started to beat so fast. It was my first real joke; I thought it was so perfect.
My grandmother went into the house with the letter and after few seconds l heard a scream. Oh. My. God. I was so scared. Only in that moment, I realized that my grandmother could have a heart attack. I was so stupid; it was a horrible joke for her! I guess (but I do not remember very well) I immediately told to my mother the reason of the scream. She ran to my grandparent’s house. My mother spoke with my grandmother, she told her about the joke (and I thought she told her that she had an asshole daughter). Then, the most difficult part: I had to apologize. I felt so guilty. Actually, I thought I was so good to write the letter (I was only 10 years old!), and the same time, I thought how stupid I was .
I apologized.
And I “do not know” the reason, but I never see my grandmother take the letters from the mailbox anymore.
From that moment, I stopped making jokes. I hate surprises. I hate surprise parties. And when I want to buy a present for someone, in most cases, I tell him/her the present or a few clues.