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Why I Let My Son Play With My Phone
A כלי שמלאכתו לאיסור can be moved לצורך גופו ומקומו. Can you move a smartphone on Shabbos? It would appear to be מוקצה מחמת חסרון כיס, something very expensive that you wouldn't use except for its intended purpose. The Beit Meir adds a caveat to this rule.