Answer:
Buzzards are widely known for their apperance being creepy or scary, When buzzards flock together they can be seen as one giant dark  circle comparing Mr.Shiftlets thoughts or head to a group of buzzards could be taken as one dark cloud in his head or multiple eery thoughts being grouped together. 
Explanation:
Im not sure if this is the answer you are looking for but this is how I would answer hope it helps!
 
        
             
        
        
        
1. Mixture--Lettuce AND tomato
2. Mixture-- Air consists of many different elements and gasses
3. Pure-- You aren't changing the substance of wood by shaving it, but if you were thinking from a makeup level it would be a mixture to begin with
4.Mixture-- There is more than just water in pond water...bacteria and such.
5. Not sure--I might say pure but I don't know for sure
6. Mixture-- Sugar AND water
        
             
        
        
        
Chapter 6, let me know if you have questions