Lego makes 3,500 different types of bricks. Nearly all of them are currently made from ABS, a virgin plastic whose production is responsible for roughly a quarter of the 1.2 million tons of carbon that the Danish toymaker emits each year.

But Lego's efforts to meet the challenge of finding a new material that is both durable and sustainable came to a head recently after the toymaker unveiled new prototype blocks sourced from recycled water bottles.

It took three years and a team of more than 150 engineers for the new blocks to reach this particular stage of development. Test run after test run was needed to find the right combination of recycled plastic (PET) and strengthening additives. Some iterations couldn't even be pulled apart with pliers, Lego VP of sustainability Tim Brooks told the BCC.

After countless trials and many errors, what finally emerged from this multi-year effort was a PET Lego brick that could meet the company's strict durability, safety, and clutch power standards.

"The biggest challenge on our sustainability journey is rethinking and innovating new materials that are as durable, strong, and high quality as our existing bricks," said Brooks in a statement. "With this prototype, we're able to showcase the progress we're making."

In 2018, Lego pledged to make all of its core products out of sustainable materials by 2030. In these and other efforts, Lego says it's responding to shifting consumer preferences, as sustainability becomes a central issue for the adults buying their products and the kids actually playing with them. Just last year, Lego pulled single-use plastic from its packaging, not in response to concerned parents, but rather partly in response to waves of letters sent to them by eco-conscious kids.

Since 2018, 5% of Lego bricks have been produced from a biodegradable polymer made of sugar cane. But Brooks told the BBC that Lego chose to focus on durability rather than biodegradability with this latest iteration of eco-friendly bricks.

Lego will make the new bricks out of food-safe water bottles primarily sourced from the United States. Lego will then use what it calls a "bespoke compounding technology" to turn crumbs of flimsy plastic into strong Lego blocks, capable of withstanding between two to three generations of play.

This "bespoke compounding" method still needs to be refined by the team at Lego before the new blocks can move onto the final phase of development.

Lego expects it will take at least a year before their new recycled bricks hit store shelves.

Asked by the BBC how many blocks Lego will make from this recycled material, Brooks said, "it's too soon to say," but added that Lego wants to use the new bricks "as soon as possible."