A temporary playground is opening along the Parkway on Saturday. During the summer a grassy lot at 21st Street will have a concrete ping pong table, gigantic foam tinker toys, and a rarity among Philadelphia playgrounds: a sandbox.

The pocket park, called "Free Play on the Parkway," represents new ideas in both urban development and childhood play.

This a sliver of land wedged between a freeway on-ramp and the Parkway. It was once the site of sculptures by Alexander Calder, and where a museum for his work had been proposed. The museum never happened, and when the sculptures were removed a of couple years ago, the site languished.

"It is mostly for dogs.," said Logan Square resident Christine Piven. "People weren't using the benches and stuff, so we wanted the community to use the space on a regular basis."

Piven, a mother of two (11 and 7 years old) teamed up with her neighbor Catherine Barrett, also a mother of two (7 and 4 years old), to organize this prototype playground that appropriates forgotten urban space for unstructured play.

Play has become a dirty word in the last 20 years, according to Temple psychology professor Kathy Hirsh-Pasek. She says free, unstructured playtime allows kids to develop critical cognitive skills.

"Now many parents hear their child say those two awful words: I'm Bored," said Hirsh-Pasek. "The reason we hear that is we never give our children a chance to have free play time where they structure their own activities."

The playground on the Parkway will be open six days a week, and maintained by the city's Parks and Recreation department. It's a prototype, lasting just until September. After that Fairmount Park officials will decide if they want to expand the idea to other pockets of the city.