Calif. — On a warm spring morning in 1976, when Beth Pratt was 7 years old, she noticed a “For sale” sign posted in the woods near her home just north of Boston.

“I asked my mom what it meant,” she recalled. “She said the land was up for sale and would soon be flattened by bulldozers.”

The next day, Pratt went door to door in her neighborhood of old elms and deep porches asking for donations to save one of her favorite outdoor playgrounds. Then she called the phone number on the sign and made an offer: $5.