Every lesson and every glossary term in the program, one box. Type what you are looking for: a machine, a term, a number you half remember from a lesson three parts back. Results open the page where the program actually teaches it.
Word starts count, so "accum" finds accumulation and "sort" finds sortation. Hits in a title outrank hits in the body. Use the arrow keys to move through the results and Enter to open one. The index is built into this folder, so search works offline, straight off the file system.