When you choose a level in a chapter, notice the "Retry" button at the lower left corner of the level. Select it and, if you click on it in the lower right corner of it, you will see a "Skip This Level" button for some levels.
ACHIEVEMENT - HOW TO UNLOCK Engineer of Goo I - All OCD Flags in Chapter 1. Engineer of Goo II - All OCD Flags in Chapter 2. Engineer of Goo III - All OCD Flags in Chapter 3. Engineer of Goo IV - All OCD Flags in Chapter 4. Engineer of Goo V - All OCD Flags in Chapter 5. Executive Producer of Goo Product - All Levels Complete. Subversive Traveler - There is a secret. The Architect of Goo - All OCD Flags.