Physics
The safest hammers are those with heads that are

Chiseled and drop forged
Alloy and drop-forged steel
Cast steel and chiseled
Welded and alloyed

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Physics
Water pipes in hilly areas often burst on a cold frosty night, because

The material of which pipes are made contracts due to cold and so breaks
None of these
Frost makes the pipes rusty, so they break
Water in the pipes freezes and on freezing water expands, so pipes break

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