The Maw

This is the first achievement that you will get. On level 1 'Crash Site', you will come to an area where you see a cutscene of Frank picking up a yum, and Maw snatching it out of his hands and eating it. You then have to guide Maw around this area to eat all of the yums. There are a couple more that can be found on the path that leads out of this area, before you get to the rock that has to be dragged out of the way. There are 8 yums in total here. If you eat all of these, Maw will grow in size for the first time.