Near the end of the Caveman Chapter, if you return back as far as you go, you can interact with a rock in an interesting way. If you press the A button while standing in front of it, it doesn't seem to do anything. However, pressing the A button 100 times will cause a hidden cave to appear. It has to be exactly 100 presses; pressing it even once more after the click causes the cave to lock, and you'll have to repeat the process.
Once inside the cave, you'll be in front of a monolith, that when inspected, accepts a single item. Using any item other than a Bone will get you blown out of the cave, with the cave locking behind you. Using a Bone, however, will turn it into a Basic Rock.
The Basic Rock, when worn as an accessory, gives a very large +50 bonus to your IQ. If it is used as an item in battle, it will scan the enemy's health, as well as sometimes lowering IQ, and disabling both arms and legs.
This is exactly what I needed! I read an FAQ, but it wasn't very clear on where the rock was. Thank you very much!
DelCaptain 10 months ago