She and several other fairies surrounded one larger rock, pinning it to the ground and holding it, so it would be as heavy as it looked rom the outside.

However, no one could know they were there and without them the rock would loose all its weight.

It was a problem that had been solved ages ago, by giving some of their magic to the rock to make it heavy. It was their last resort, only used when somebody lifted and took it, but it worked quite well. Even on the larger rocks.