A class from Two Rivers Public Charter School tackles multiple construction projects at the "Play Work Build" exhibit. ( KEVIN ALLEN )

If you’re a fan of building blocks, you probably have a set the makes a castle, a fighter plane or a hospital. There are step-by-step directions to get to the finished product. But what if the blocks came with no directions and no theme? What would you make?

If you’re curious, head to the National Building Museum. The museum’s “Play Work Build” exhibition is filled with hundreds of bright blue blocks made from a dense foam. There are little blocks to build on a table, and big blocks to stack several feet up from the floor. There are even virtual blocks. All you need to bring is your imagination, and you can build anything.

The blocks, which include arches, bends, hinges and a round shape called “little cheese,” have inspired kids to make a huge range of creations.

“I would like to build forts,” said Alex Rosenbaum, 10, who with his parents and brother Jacob was on his second visit to “Play Work Build.” Alex said he preferred the large blocks and would recommend the exhibition to any kid who likes to build.

“I could stay here for hours,” he said as he and Jacob, 7, began to construct a square fort that grew to four feet tall.

Timari Brown of Two Rivers Public Charter School stacks the smaller blocks in the exhibition. ( KEVIN ALLEN )

Their fort got the attention of younger builders nearby who were interested in helping.

The ideas of starting from scratch and working together is just what New York-based designer David Rockwell had in mind when he created the blue blocks.

Rockwell, who has two kids, came up with the idea of creating a playground in Lower Manhattan, the area of New York City damaged in the September 11, 2001, terrorist attacks.

“Most of the playgrounds in the city . . . didn’t encourage the taking apart and putting together,” Rockwell said.

So Rockwell and his design group dreamed up a set of blocks that come on a cart and could be used inside or outside. They tested it at a New York recreation center.

“What we discovered is [that] kids would play more with that set of blocks than with anything,” Rockwell said, “because every day they could build something new.”

The sets, called Imagination Playground, can be found at 600 playgrounds worldwide, including Washington’s Two Rivers Public Charter School and the Beauvoir School.

When the National Building Museum’s curators were looking for something kid-friendly to replace the popular “Lego Architecture” exhibition, they went to Rockwell. His company donated the playground-size blocks and created smaller ones especially for the museum. Rockwell also designed a huge screen that fills with falling virtual blocks. If you stand on a special mat in front of the screen, cameras capture your shape and it appears onscreen. Virtual blocks then magically fill that shape and continue stacking up to the ceiling.

Isabella Valles, 9, of Columbia said her favorite part of the virtual blocks wasn’t watching them rise in a stack. It was what happened when she moved around on the mat.

“It was fun to make them fall down.”

READ: If you go

Christina Barron