In essence to activate charcoal you heat it to something around 750 degrees C with steam. This causes the charcoal to sort of unfold giving the a tremendously increase in surface area. Since charcoal removes impurities by having them adhere to its surface. This increase to the surface when activated allows it to remove much more contamination.
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When in danger or in doubt
run in circles scream and shout
RAH

And always remember TANSTAAFL