Bur Oak Acorn, Walnut-sized Nut

I spotted the curious-looking bur oak acorn just before Halloween. The acorn had a bushy cap, like a short-dreadlocks hairstyle. The acorn itself seemed about the size of a walnut. After I took it home, I found that the cap easily detached. The composite shows the acorn and cap with measuring stick. Read the entire article and comment there, please.