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For fleas (and cockroaches too): boric acid crystals, it's a fine powder. Don't put it out thick, just a dusting. In those quantities it is entirely harmless to humans and pets, and children.
Soap keeper--can you imagine the germs population living in that sock holding th old soap shards? Yekk. But some were fun ideas indeed.
Okay, okay. Go ahead and use it. Soap is not a disinfectant, but okay. It's the sock that I'd be concerned about, just like a kitchen sponge or any washcloth...didn't mean to upset anybody.