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Regardless of whether your mom is present in your life, your dad almost certainly has an understanding of periods.  It's possible that he learned about it in school. He might have some knowledge about it from women in his life, like his mom, sisters, aunts, and so forth. Some people communicate better in writing than in speaking, so use that to your advantage. Come up with an outline of what you want to tell him.  “Dad, you should know that I started my period.” “There's been a change in my life lately, and it's that I got my period.” “I felt uncomfortable telling you in person, and this feels easier to me.” “I was afraid it would get too awkward if I told you in person.” “I need feminine products, so can we go to the store?” “Can you give me money to buy pads?” “I need a pain reliever for the cramps I've got.” This could make talking about your period less uncomfortable and make it funnier.  “Aunt Flow” “Monthly visitor” “Red wave/red sea” “Crimson wave” He wants  to be able to help you, and he wants what is best for you.  “Thanks for understanding and supporting me, Dad.” “I appreciate that I could find a way to tell you about this.” “Dad, thank you for always being there for me.” You do not need to write him a long letter all about how you got your period. Keep it short and to the point, and use an appropriately-sized sheet of paper or card to do this. You want to make sure you tell him what is going on and what you need from him, which you planned in Step 2.  Use clear, legible handwriting so that your father can read your note. Start the note with a greeting such as “Dear Dad” or “Hi Dad.” End your note with a closing such as, “Love, Susie” or “Thank you, Morgan.” If you chose a small notecard for your note, it probably has an envelope. Otherwise, you can fold up your paper and slide it into a mailing envelope.  You can seal the envelope by licking it yourself, or you can use an envelope moistener or a damp sponge. On the front of the envelope, address your dad by writing, “Dad,” “Daddy,” or “Father.” Consider rooms in the house your dad frequently visits or items he frequently uses, such as his briefcase or laptop bag.  Avoid leaving your note in a public place where someone else might pick it up. Leave your note where he will see it, such as on a counter near a gadget he often uses, inside his briefcase, or on his desk. If your dad does not approach you about the note, it is a good idea to ask him in a day or so if he read it. (Be sure to ask him in person). This way, you can know that he knows that you got your period and can help you get supplies. Be confident when you follow up with him. Ask him, “Did you get my note?” and let him know that you need to go to the store if that is the case.
Keep in mind that your dad knows what menstrual cycles are. Plan a draft of what you want to say to him. Make a suggestion that you come up with a code word for your period. Thank your dad for supporting you and taking care of you. Find a nice piece of stationery paper or a small notecard. Write your note to your father. Slide your note into an envelope and seal it. Leave the note for your dad where he will find it. Follow up with your dad to confirm that he read your note.