“It reveals that sometimes it’s the most mundane and invisible that’s the key to seeing things in a new way,” the principal of Invisible Design, Harry West, told attendees of the Fortune and Wallpaper* Brainstorm Design Conference in Singapore on Wednesday.

Tetra Pak noticed its cardboard packaging was losing market share as more people switched to plastic bottles from the cartons and juice box-style containers for which the company was best known.

Tetra Pak engaged design firm Continuum, where West worked at the time, to help them claw back customers. As an added complication, any solution had to fit on a small rectangle of cardboard on the top of the packaging.

West said they started with a seemingly very simple question: How do people drink?

“Surprisingly, Tetra Pak—which is a multibillion-dollar company and its very existence depends on people drinking—had never asked that question,” he said.

Continuum put video cameras in the bottom of bottles to get a closer look at how people drink. The discovery: Not everyone uses the same technique. Some suck (by forming a seal around the entire lip of the bottle), some pour (by tipping the water into thier mouths without forming a seal), and others pull (by forming a seal between their top lip and the surface of the liquid).

This approach informed their solution, which was a resealable plastic spout that sat atop Tetra Pak’s cardboard packaging.

“Now that we understand the rules, we can understand how to design the ultimate interaction between the lips and the opening and the closure that will deliver that,” West said.

The new packaging was first offered to customers in Saudi Arabia before being used globally, West added. It’s now used for hundreds of different beverages and by millions of customers around the world.

For more coverage of Fortune’s Brainstorm Design conference, click here.