Cracking you knuckles isn't bad as long as you push on them
lightly.
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Cracking long-chained hydrocarbons produces two things which
include alkenes and alkanes(shorter chains), including hydrogen
through catalytic cracking.
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Boiling. Just as cracking is a type of breaking, kettle is a type of device used for boiling water or other liquid.
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Cracking is correct, as in cracking an egg.
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Endothermic because the egg is absorbing the heat.