Mouth agape, 4-year-old Sarah Farrington watched in anticipation. The canister popped across the room, becoming a miniature bottle rocket.
At the next station, children stirred paper cups filled with glue, green food coloring, and a borax mixture - "a very complex polymer chain that loves to stick together," institute staffer John Miller explained.
The young scientists stirred and stirred until the contents reduced to a glob of green slime. One boy raised the glob in front of his face, looking for a split second as though he might put it in his mouth.
"It's squishy and slimy," said Josiah Fisher, 9, of Red Hill. He experimented a little and added, "It's not bouncy."
Next up: two watermelons on pedestals, bound in the middle by rubber bands. Each visitor added three more bands, until - once the total reached 200 - the pressure caused the watermelon to explode.
"Don't be scared! It's not going to blow up - yet," Joe Gallagher said, waving children up to the pedestals.
Being not immediately messy, the watermelons were a quick pit stop on the way to the next table.
Maayan Shahar, 11, stood on a stool surrounded by a moat of soapy water. As a staffer encased her in a huge bubble, Maayan tried to stay still, her eyes widening as it passed her head. It popped. She laughed.
"It's like being inside a big -" she said, then paused, trying to think of the word - "wall. A transparent wall."
A lot of space was reserved for bubble trays, where children could make their own orbs, large and small, singles and multiples, using tools and by hand.
Some were bubble-blowing pros; others needed a little coaching.
"I'd say slower is a little bit better, and you've got to turn the wand along with it," a staffer advised, demonstrating a graceful twist of the wrist.
Nia Lloyd, 8, of Sicklerville, was a natural. She pivoted her whole body, closing up the contours of a 2-foot-wide sphere that floated and wobbled for a couple of seconds before hitting the ground.
And then, with a loud boom, the sleeper exhibit became the favorite. Watermelon was everywhere.
"There go my brand-new shoes!" said Tyler Chew, 12, of Bristol. His white shirt splattered with pink pulp, his hair smelling like watermelon bubble gum, Tyler was proud to have busted the big guy.
"I could feel it caving in and then it just -" he said, waving his arms up. "I probably shouldn't have worn a white shirt."
Contact Jessica Parks at 610-313-8117, email@example.com, or follow on Twitter @JS_Parks.
The "slimy, oozy, gooey" Mess Fest will have a second session Sunday, then two more next Saturday and Sunday. All run from noon to 4 p.m. at the Franklin Institute, 222 N. 20th St., Philadelphia. Admission is $12.50 for ages 3 to 11, $16.50 for ages 12 and up.