the character was named and based on his real daughter, trixie. first book was published. ♪ >> reporter: at 4 when she voiced the character. >> it's all true -- >> reporter: now she's 15. >> you know, being the real trixie is kind of contained in one part of my life and that i go and visit places like this, and i'm suddenly at this museum where all my dad's stuff is being held. i'm like, this is a thing. >> reporter: a thing parents and kids are willing to wait in long lines for. >> want to hear a story? >> reporter: especially to hear willems voice the characters from the final "elephant and piggy" book. >> we thank everyone. [ laughter ] >> reporter: it's called "the thank you book." this cover is the 25th in the series. >> over the years, piggy's ears have gotten bigger because she's aged. and joe's ears have gotten bigger and longer. i want to go back to projects that really freak me out, that i think can i do this? and then i can use that energy of being terrified to push myself to do something that hopefully will be meaningful. >> reporter: