What gesture of friendship cements Miss Maudie and Scout's relationship?

As Scout, Dill, and Jem continue to grow up, they also begin to change. One of the most evident changes is that Dill and Jem start to bond more as boys, leaving Scout behind in their games. As a result, Scout ends up spending more time with her neighbor, Miss Maudie, who likes to garden and has a very amiable way of treating children. Scout was not too familiar with Miss Maudie as a child, but, as part of her own growth into young womanhood, she is now warming up to the dynamics of female friendships. After all, why not be friends with Miss Maudie? She makes the best cakes in town!
As time went on and Scout spent more time with Miss Maudie, she was able to get more insight about the lives of the Radleys. She finds a lot of information that serves as a way for her to understand that there is much more to the rumors and myths surrounding the family. Scout learns a lot from Miss Maudie. However, the ultimate act that cements their friendship is when Miss Maudie opens up and shows Scout a little secret: her dentures.

[When] she grinned she revealed two minute gold prongs clipped to her eyeteeth. When I admired them and hoped I would have some eventually, she said, “Look here.” With a click of her tongue she thrust out her bridgework, a gesture of cordiality that cemented our friendship. 

Notice that Scout says that it was an honor to see the bridgework. It is a gesture that indicates that there are no secrets between them and that they are friends. It is clear that this was an important moment in Scout's life: she remembers it as a symbol of a long-lasting friendship.