thumb tack (40a) + mouth 口 (5) = stone 石

Notice how the thumb tack has slid off to the side — it is not fixed to the top of the character (as in, for example, 百, Character 40).

You’re trying to fix up a notice saying “Slippery Stones” with a thumb tack at the mouth of the cave, but (ironically) it keeps slipping on the stone. / [No pronunciation needed]