"How about this?" I asked, taking a small coil of copper wire from my desk.
"I guess that won't stick," said Charlie. "Because that's copper wire, and the magnet doesn't seem to pull any thing that isn't iron."
Much to Charlie's satisfaction, the magnet did not pull the copper wire. Then I took up two stones, one rusty red, the other black, and said: "What about these?"
"I guess they must have iron in them too," said Charlie. "Have they?"
"They have," I replied. "They are iron ores from which iron is made. Why did you think there was iron in them?"
"Because they wouldn't have stuck to the magnet if there wasn't."
"Quite true. So you have learned another very important fact. Can you tell me what it is?"
"The magnet pulls iron," said Charlie.
"Good," said I; "and it is also true that the magnet does not pull—"
"Things that are not iron," said Charlie.