Occasionally when you are walking through a field, you come across flints that have been in a fire - they have a characteristic light blue-grey colour and their surface is crazed like the glaze on old plates.
The usual explanation for these flints is that they are "pot boilers" - stones placed into the fire to heat up and then transferred to a pot of water to boil it. On long winter evenings at Archaeological Society meetings in Victorian times, members would argue endlessly about whether this or that newly-found rock could be a pot boiler - this may be one of the sources for the phrase's modern idiomatic meaning of endless and pointless rumination about a subject of little or no real merit.
No doubt this is one way to heat water, though not a very efficient one. But I wonder if the the many burned flints could represent some other activity? It has been seriously suggested that they might be the remain of a prehistoric saua bath! Heating flints can certainly alter their brittle qualities as a material so perhaps they wre experiments at "annealing" the stone.