Legos just click. If you've ever played with a competing brand of "interlocking plastic bricks," you know that Lego's big advantage is their solidity, their seemingly infinitesimal tolerances that make sure every piece fits just so with every other. The seams turn invisible. The secret to that tight connection (and how painful Legos are to step on): plastic. Specifically, a very tough plastic called ABS, or acrylonitrile-butadiene-styrene, three polymers derived from petroleum. So last month, Lego announced that it would launch, later this year or next, a Sustainable Materials Centre—100 engineers, chemical engineers, and materials experts all trying to find an eco-friendly replacement for ABS and other ingredients in the company's toys.

Finding those replacements will be tougher than getting a one-by-one piece off a wide base plate. (That's hard.) ABS is great. It’s precisely moldable; every Lego block has to be identical to others of its type to within 4 microns, from batch to batch, year after year. ABS also takes color well, so a wall of red bricks looks the same across its entire surface. You can print on it, it’s durable—important for a toy that gets passed down through generations—and, most of all, ABS can create what Lego calls good “clutch” power, the ability to stick to other bricks until kids pull them apart.

Plus, what does "sustainability" mean in this context? Right now, companies can define that word pretty much however they want. No carbon emissions cutoff exists to qualify a material—and even if one did, it’s notoriously difficult to tally up those emissions. A sustainable material could be renewable or recyclable or both (or neither).

To Lego, sustainable means changing where their plastics come from—the raw materials, or feedstocks. "The feedstock for plastic can come from many places that are not fossil based—bio-based, renewable or even recycled sources," says Tim Brooks, Lego's senior director of environmental sustainability. Part of the new center’s mission will be to test new feedstocks, including ones that are made out of plants. The center will also work to find a material that itself is recyclable, and that overall will have a lower carbon footprint than its current plastics—accounting for the energy that goes into shipping the raw materials to the plant, running the machines that assemble the toys, and the materials themselves.

At a meeting last year, senior project manager Allan Rasmussen spoke about some of the company’s earliest efforts to phase out ABS bricks by 2030. He said they had already tested an impact-modified polylactic acid—a plastic produced from corn—that was “very, very close” to what they needed.

But if you've ever eaten something hot with a compostable spork, let's say, you might have noticed that current bio-based plastics almost uniformly suck. It’s hard to make a resilient plastic out of plant starches or carbon compounds spit out by bacteria. In the case of polylactic acid, Lego ran into problems with the blocks’ ability to click and stick. A few weeks after molding, the plastic had lost its original shape.

And it’s not just the iconic bricks that Lego will have to redesign. The majority of Lego’s output is in ABS—it's the most common material used to build more than 60 billion bricks every year—but Lego uses a lot of other materials these days, too. Technic pegs that need high levels of friction, car wheels and axles need to slide easily against each other, pneumatic air hoses need to be flexible, car tires need to be compressible...and that doesn't even begin to get into fabrics. Making things even worse for the greenies at Lego, the company packages all of those parts for sale in plastic bags nestled in plastic shrink-wrapped boxes.

In fact, packaging might be a better target for sustainability. All that stuff gets thrown away. But when's the last time you heard of someone sending even one Lego brick to the landfill? Maybe it’s Lego customers who can do the most good—by passing down sets they already own.