Help! Been looking for the answer for years!

I got this conundrum from a web site that offers no answers and no way to contact the owner. I've posed this to almost everybody I know and nobody has an answer.

Two towns are separated by a field. One fine Saturday morning all of the kids came out of their houses from both towns and ran into the field. Suddenly they all stopped, looked at the field in horror and ran back into their houses crying. The field was empty except for two piles of stones in the middle of it. What happened?