Most of the questions at the Philadelphia Science Festival's carnival Saturday were perplexing, a few downright gross, and while some discussed salad dressing and slime, others were vital to the future of mankind.

"Yes, absolutely, it's a real brain. This is a science festival," a man with the title "Dr. Brain" Dude told a little girl at the Cooper Medical School of Rowan University booth.

The sign above the brain in the glass jar asked, "What's inside your skull?"

Thousands of people and several dogs crammed the Ben Franklin Parkway for the carnival, the festival's final event. They pushed baby strollers past 150 sponsored booths, each posing questions that couldn't be answered in one or two words.

"Can you control a vortex?"

STEM Scouts, a scouting program that focuses on science, technology, engineering, and math, instead of tents and hiking, say yes. You need a shower curtain, ping pong balls, a trash can, and something called "Bernoulli's principle."

"What's amazing about horseshoe crabs?"

For one, their blood is a milky blue, but if Pamela Green, a professor at the University of Delaware, had to pick one thing, it's how ancient the peculiar seashore staples are.

"They are so resilient," Green said. "They haven't changed all that much in hundreds of millions of years."

The festival was started by the Franklin Institute in 2011, and Saturday's carnival was the culmination of a week's worth of events, most of them free. Last year's festival was held at Penn's Landing as the NFL draft had taken over the Parkway.

Children's Hospital of Pennsylvania asked "How do you stay healthy?" and some of the answers were depressing, albeit honest. For instance, drinking a Starbucks Caramel Frappuccino is equivalent to eating six glazed doughnuts. Volunteers had put the amount of sugar in soda and even sports drinks into plastic bags.

They were hefty.

"Mommy drinks Snapple because I'm bad," Marina Williams, 31, told her daughter, Bella, 5, of Philadelphia.

The chemical giant Dow didn't pose a question, but what the company built on the sidewalk by the Franklin Institute prompted many. First guess would be a sandbox filled with Pepto-Bismol, but Stacey Barnaby, a Dow chemist, said it was called Oobleck, a reference to a Dr. Seuss book.

"It's 350 pounds of corn starch with water and some food coloring," Barnaby said.

Children ran across the oobleck — not quite liquid, not quite solid — and their feet didn't sink.

"Its almost like quicksand," Barnaby told the crowd.

Sophia Paneiro, 9, of Haddonfield ran through the oobleck a dozen times, barely any pink stuff on her bare feet when she was finally done.

"It sort of feels like clay," she said. "Wet clay."

Biology students from Rowan University asked, "What reptiles can you find?" Most were in containers on a table, but an eastern rat snake was wrapped around Callan Tweedie's arm.

A few people screamed when they realized it.

"You can touch him if you want," Tweedie, 21, told a group of reluctant teens.

The Academy of Natural Sciences posed a question that bordered on the philosophical: "Can you hack it?" Inside, children were tasked with moving one plate of marbles to another in one minute with a pair of chopsticks. It proved difficult, with marbles rolling all over the table.

One boy, Sid Shankar, 8, took one chopstick and slowly swept all the marbles from one plate to another, truly hacking the challenge. He wants to invent things when he gets older, his proud father said.

"Mostly electrical stuff. I want to take stuff that runs on electricity and make it run with other energies," the boy said. "I want to make the world better."