Starch is present in saliva.
I don't know which food choice you made so I can't explain why you got the results you did.
My answer: Because the actual thing you might be modelling might be too small or large to replicate, (for example you cannot draw a plant cell TO SCALE because it’s way too small to see with a bare eye).
And if the item is too big, you won’t be able to fit it in a small space; if you were to draw it or create it in 3D.
I hope this helps!
Sorry if the response is too short, try rewording if you need to :)
14. Dermis
15. Chemical
17. Virus
18. Antibodies
19. Gas exchange
20. Ligament
Hope this helps!