Let them dry in the sun a few days and the cap comes right off. Usually the shell will have a hairline crack you can pry open, but a mallet w/moderate force works.
Thanks for the acorn cap tip! I have a small bowl of the Burr Oak acorns as ‘decor’ in the Family Room. They are rather pretty.
Our driveway is lined with Burr Oak, so when I go to get the mail later today, I’ll see what’s fallen so far. ;)