One day, a student entered a room where he found two large glass jars filled with clear liquid. As he approached the table, the teacher called out to him, “Be careful! One of those jars is filled with water, but the other one is filled with acid, and you could be harmed severely if you were to get it on you!”

The student stopped in his tracks. “But teacher,” the student called out, “how can I tell which one is filled with water and which one is filled with acid?”

“To be sure,” the teacher answered, “both liquids might appear at first to be very much the same.” He then handed the student a large iron rod and winked. “But you can learn their true natures immediately by watching what they do.”

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