The first world gently introduces you to the organised chaos of goos and then it gets trickier from there. The real difficulty is getting Obsessive Completion Distinction (OCD). Some of these rely on twitch responses and reflexes but some require you to be economical with your moves and careful about goo positioning. It's very satisfying sussing out the best way to get that accolade and in some cases I had to give in and move on!

World of Goo
 

Once you've played the game, there are over 50 fan-made levels to add in available from goo-fans.com. They should keep you busy for a little while!

REALLY?! Wow thanks! I finished it ages ago and have been craving more World of Goo-dness for ages. Sorry I couldn't help that pun...