Bert's New Collection is a 1988 Sesame Street storybook featuring Bert and Betty Lou, designed to teach about sorting and "what belongs together." It showcases Bert's penchant for collecting, from his famous collections of bottle caps and paperclips to lesser known passions, such as argyle socks, sponges, and hats with feathers.
Bert even has a song to sum up his hobby: "Collecting rulers is groovy! Collecting sponges is mellow. Erasers, paper clips, bottle caps, too! Oh, I'm such a happy fellow!" Apparently this dates back to childhood, with Luis starting Bert on collecting rulers and Herry Monster introducing him to Figgy Fizz bottle caps.
When Betty Lou admires Bert's collections, Bert decides to teach her how to start her own collection. Since Betty Lou has a single brown button in her pockey, an eager Bert declares her a lucky duck and forcibly drags her out to begin a brown button collection. While Bert is singlemindedly focused on brown buttons, though, he overlooks other collecting opportunities. By the end of the epic, they still have one brown button, but Betty Lou has amassed an enviable collection of blue items. -- Muppet Wiki