5.md

1. Add a _saved Set to RandomWordsState

class RandomWordsState extends State<RandomWords> {
  final List<WordPair> _suggestions = <WordPair>[];
  final Set<WordPair> _saved = Set<WordPair>();   // Add this line.
  final TextStyle _biggerFont = TextStyle(fontSize: 18.0);
  ...
}

2. In the _buildRow function, add an alreadySaved check

Widget _buildRow(WordPair pair) {
  final bool alreadySaved = _saved.contains(pair);  // Add this line.
  ...
}

3. In _buildRow() you'll also add heart-shaped icons

Widget _buildRow(WordPair pair) {
  final bool alreadySaved = _saved.contains(pair);
  return ListTile(
    title: Text(
      pair.asPascalCase,
      style: _biggerFont,
    ),
    trailing: Icon(   // Add the lines from here... 
      alreadySaved ? Icons.favorite : Icons.favorite_border,
      color: alreadySaved ? Colors.red : null,
    ),                // ... to here.
  );
}