What behavior of Ellie’s makes Debbie worried?

W: I’m a bit worried about Ellie. She’s nearly six and she still sucks her thumb!
M: Oh, Debbie, you needn’t worry too much about her. I’ve got a friend who has sucked her thumb for twenty-nine years’
W: You’re kidding!
M: No, honestly. She only does it when she’s really tired, but you can imagine how strange it looks on a grown woman.