It's a rooster spur!
Look for the white rooster inside the clear marble.
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The second spur is on the "Jingles Deputy" badge near the lower-middle of page 20. It is hard to see on this page, but the same badge is laying flat on page 29, and you can clearly see the spur in the center of the badge.
The cat is riding a yellow bike at the top of the page
There are many HEV configurations and design options that can be grouped in three categories: series (range-extending HEVs), parallel (power assist HEVs), and dual-mode HEVs.
on the photos towards the bottom left of page 22
Hint 1 : It's on page 8. Hint 2 : The octagon is red. Hint 3 : It comes in the form of a roadsign.
Go back and forth from page to page, doing one problem on one page and then going to the other page to do one, then back to the first page, then back to th second page, and so forth. When you are finished, both pages will be done!