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One day when the boys came home from school they found a man at work putting some new panes of glass in the window.
“Oh, this is luck,” said Fred; “I’ll ask him for a piece of his putty, and then we can have a chat about it.”
“Did you see the man rolling the putty about in his hands, Norah?” he asked, as soon as they sat down.
“Yes,” said Norah, “and I knew at once that putty must be a plastic substance.”
“Quite right,” said Fred. “Now I will show you some of the things we have learned about putty to-day. Look; I will make it into a little cup, and then fill it with water, just as teacher did.”
The children watched it for some time, and then Willie said, “I am sure Norah can tell us all about it now.”
“Yes,” said Norah, “the water stands in the cup, as it stood in the cup of moist clay. The putty is not porous—water will not pass through it.”
“Quite right,” said Will. “Teacher told us to say that putty is waterproof.”
“Now let me have a turn,” said Fred. “But what a mess our hands are in. How this putty sticks to our fingers.
“I say, Fred,” said Will, “do you think Norah can tell us why the workman was glad to have sticky putty? We know, don’t we?”
“Why, I suppose it is because he wants it to stick fast to the wood of the window-frame,” said Norah.
“That’s my clever little sister once more,” said Fred. “Now, Norah,” he went on, “we have been moulding and working up the putty, because it is soft and plastic. But try and work up this piece.”
“I cannot press it,” said Norah; “it seems as hard as a stone.”
“Yes,” said Fred, “and so will this piece be if we leave it for a few days. Now let us see why putty is so useful.
“It is soft, plastic, and sticky when we use it; it soon becomes almost as hard as stone. It is waterproof, and will not let the rain come in.”
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