The forgotten birthday

"Ha, but that's actually good that you were on top of it! You see, I have a problem with my son Peter! He recently turned 18! Unfortunately, I've forgotten his birthday...".

"Oh!" came from Frank sympathetically.

"Yes! And now, of course, I have to make up for it somehow! Fortunately, I also have an idea how! And only you can help me!" Walter grinned mischievously at him.

"Okay... What do I have to do?" came from Frank, who didn't yet know what he had let himself in for.