The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
A "substring" is a group of consecutive letters in a word, like STR in STRING, ABSTRACT, or CONSTRUCT. On each level, you'll be given a new substring and will have to find words containing it. The rules change from level to level, switching between fast-paced bonus rounds and challenging ones with crossword-like clues, so stay on your toes!