I chose 'whistle' as a medium difficulty word because:
1. It's 7 letters long, fitting within the 4-10 letter requirement
2. It contains some common letters (e, t) but also less common ones (w, h)
3. It has an unexpected letter combination 'stl'
4. It's a real English word that most people know, but not one used in everyday conversation
5. The placement of 'h' after 'w' might not be immediately obvious to players