Explain what is meant by the terms grain and grain boundary?

I need help with this question as when I type it in all ive got were guns showing up but this is engineering or:I need help with this question as when

I need help with this question as when I type it in all ive got were guns showing up but this is engineering

or:I need help with this question as when I type it in all ive got were guns showing up but this is engineering

or:If you look at wood you can see that the cells all lie in a common direction. The wood is strongest in that direction, and is easily split or broken by a force applied across the grain. Diamond has a grain and in one direction it is so hard that it can't be scratched even by another diamond. But if you scratch glass with the point of a brilliant cut diamond, you run the risk of breaking it. The same is true in metals but harder to see. Various crystals of metal* and other ingredients form at specific temperatures and that determines a lot of characteristics, all generally called \"temper\". A grain boundary is where two grains meet and is considered a defect in the crystal structure.*Primarily iron, but silicon is also of interest.

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