Amazon Spheres just unseated the Central Library as the architectural king of Seattle. The Spheres, two ball-shaped greenhouses, opened in February as a place for Amazon employees to go work and think. The building is a glass-and-steel marvel, filled with rainforest greenery, trickling waterfalls and plenty of comfortable seating. If the throngs of selfie-taking visitors were any indication, the Spheres are now the place in Seattle to sightsee.   The Spheres opened to the public two Saturdays a month in April. Want to visit? The hottest tickets in town are free, but they go fast. You can reserve them one month before the public Saturday, so mark your calendar 30 days in advance. On the public Saturday we visited, the Spheres were swarming with families. Babies in carriers and photo-snapping parents fanned out across the four levels of indoor gardens. The kids loved the huge fish tanks and a room shaped like a bird’s nest. At the center of the building is a 65-foot-tall living wall, with more than 100 species of plants attached to vertical mesh.   But before you book your tickets, be honest with yourself. If your kids are escape artists who can’t keep their hands to themselves, this is not the place for you. Parents, the number one rule in the Spheres is do not touch the plants. You should also know that in this lofty, open space, the only thing blocking the view is thin wire mesh. If you’ve got climbers, keep a tight grip on them. Railings are adult-waist high, and it’s a long ways down from the fourth floor. Parental warnings aside, the Spheres are absolutely breathtaking: modern architecture entwined with lush, tropical greenery. The environment is modeled after a cloud forest, which naturally exist at high elevations near the Equator. Like the rainforest, plant life is dense, but unlike the rainforest, temperatures peak at 75 degrees. The Spheres are home to about 35,000 individual plants, including lots of mosses and ferns. (The only fake plants are the fiberglass stumps hiding the air vents.) The largest plant is a 55-foot-tall fig tree nicknamed “Rubi,” so big it had to be lowered by crane through the ceiling.     The Spheres are sandwiched between two towering Amazon office buildings. On the other sides are scaffolding and cranes – more Amazon office buildings in the works. Amazon employs more than 40,000 people in Seattle, and the Spheres are intended to be used as a workspace during the week. On a public Saturday, you get a wrist band at the check-in desk and you can stay as long as you like. Parents, note that no outside food or drinks are allowed. The café inside, Rana e rospo, is only open weekdays. The coffee shop on the second floor, General Porpoise, serves great espresso ($3.25) and doughnuts ($4.25), but not great kid snack food. Fuel up before you go – maybe try Amazon Go next door. It’s the new grocery store without any checkout lines. Download the app, and items are directly charged to your Amazon account. Blue-shirted Spheres docents were scattered throughout the levels to welcome visitors and answer questions. They couldn’t have been any nicer. At the photo op on the fourth floor, the Spheres docent graciously helped take group photos for family after family. Other rules to know: No outside plants. No flash photos. No pets. No bags bigger than 12 inches by 12 inches. Adults need to bring a photo ID to check in. My story is on ParentMap here.