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Used soap bars get dry and then crack. There are two options, throw away the remains or make a mini-soap-ball that will keep cracking if you don´t keep the pressure and humidity.
A simple shape change from the manufacturer, a dent in all soap bars, will allow new ones to be filled with the remains of used ones providing a helpful solution, ad infinitum.
I´ve proposed this idea to Unilever ↗ and they reject it; they know nothing!