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Inside the popcorn kernel gains heat, causes the inside to expand, so it pops.

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Group 3 : the air in the ballon expands because of the heat from the fire

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Ans:when you heat the popcorn the inside expand put presure on the skin which eventully pop

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The air in the hot air balloon  expands when the burner is on.

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Group3 : in the old times, it is wood but now it is metal and wood is a poor conduters of heat and the metal is a good conduters of heat

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Group 5 :The corn gains heat causing it to expand. Then it pops!

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Group 5 :The envelope (balloon) gains heat from the burners under it causing it to expand.

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Because if the power line is already straight then in winter it will contract then the power line will be broken

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O

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Hot air balloon work because hot air rise.By heating the air inside the balloon with the burner, it become lighter than the cooler air on the outside

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Air in the balloon expands because of the heat from the burner

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Once the corn seed gained heat so it expand

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There are gaps, because when  it gains heat it might expand, so when there is no gaps it might get out of shape.

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To let the metal cool from the heat from the train's wheels

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If they is no gaps between the rails on a sunny day the rail will expand and crush each other

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When it is winter the surrounding is cold, so if it contracts it will not pull each other.

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The heat from the microwave

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Because when the train runs along the track the track gains heat and expands if there are no gaps the track will push each other and the train cannot run.

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Because when the train runs along the track the track gains heat and expands if there are no gaps the track will push each other and the train cannot run.