How do I reuse ceramic bathroom tile?

I have tiles in both bathrooms(builder beige) that have fallen off and some broke. I would like to combine and reuse tiles in one bathroom and only have to buy new for one bathroom. How is best way to clean the backs off and reuse? Tired of family telling me it can't be done. Any suggestions most appreciated. Putting up new mold resistant sheet rock, so backing will be new, no old glue.

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