I just came up with another of my favorites.
I started to make my family tree online because of a special book I have. A very old nursery rhyme book has been on my book shelf for a long time. My grandma gave it to me a long time ago and I've always known that it's VERY old but I wasn't really interested until recently. There's a small note on the front pages that says who was given the book. The first person who was given it died when she was 3 or 4 years old. This got me interested in genealogy. My online family tree has now more than 100 people!!