“Let us see what you have in this pile,” I said, looking at the first little heap; “keys?”

“Trunk keys,” said Charlie.  “It doesn’t pull door keys.  I tried ever so many.”

“Try this key,” said I, taking one from my pocket.  “This is a trunk key.  See if the magnet pulls it.”

“No-o,” said Charlie, thoughtfully, “it doesn’t; but it pulled all the rest of the trunk keys I could find.”

“Try this key to my office door.”

Charlie tried it, and to his great amazement the key stuck fast to the magnet.

“Surely,” said I, “it pulls some door keys, and fails to pull some trunk keys.”

Charlie was more puzzled than ever.  He looked at the keys, thought a moment, then picked up my trunk key, and said:  “This key is brass; the rest are iron.”

“That’s so,” I said.

“And all these door keys that the magnet didn’t pull,” he continued, “are brass, too.  Perhaps it can’t pull brass things.”

“Suppose you try.  But first see if there are any brass things that the magnet pulled.”

Charlie looked them over.  Then we tried the casters of my chair, and all the other brass things we could find, none of which the magnet would pull.

“There’s no use in trying any longer,” said Charlie.  “It won’t pull brass.”

“Then, there’s another matter settled,” I said.  “The magnet does not pull brass.  Is there any thing else it does not pull?”

“Wood,” said Charlie.  “I tried lots of pieces.”

“Any thing else?”

“Stones,” said Charlie, eagerly.

“What are these?” I asked, holding up a couple of heavy stones he had put among the things the magnet pulled.

“I guess I put those there by mistake,” said Charlie, testing with, the magnet a number of stones in the other pile.

“Try them,” I said.

“O!” he said, as the magnet lifted them; “I forgot.  It does lift some stones.”

“Well, what else have you in that pile of things the magnet did not pull?”

“Glass, leather, lead, bone, cloth, tin, zinc, corn, and a lot of things.”

“Very well.  Now let us see what the magnet does pull.”

“Iron keys,” said Charlie, “and nails.”

“Here’s a nail in this other pile.”

“That’s a brass nail.  The magnet pulls only iron nails.”

“What else have we in this pile?”

“Needles, hair-pins, screws, wire—­iron wire,” he added quickly.  “Brass wire doesn’t stick, you know.”

“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?”