Anyone who's familiar with the Grow series doesn't need any explanation about this game; I'll just say the quality of this episode is excellent - possibly the best one yet - and there's a secret ending. Oh, animations in this one can be very long, but you can skip them if you want.
'Grow' games are very simple: you have a set of buttons you can press in any order you want, and depending on the order you choose, different things will happen. Every game has a different setting (a valley in this case), but they all share the same graphical style, funny characters and animations. The objective of the game is to make the place 'grow' as much as possible, but you can have fun trying random growth patterns to see what happens; figuring out the correct order is only part of the fun.
You can play this game
here (requires Flash).