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Thousands of Americans are receiving seeds in the mail wrapped in packages with Chinese characters. The mystery seeds, which the recipients did not order, have triggered warnings from agricultural authorities who say “do not plant them.”

The USDA and FBI are still investigating the packages—which reportedly include seeds for grass, cucumbers, and melon—but the growing consensus among law enforcement is that they are part of a “brushing” scam.

The term “brushing” was unfamiliar to most Americans until news about the seed mystery. Here’s what it means and how the scam works.

What is brushing?

Brushing is a shady way to boost the popularity of an online merchant by sending out unsolicited packages containing items of little value. The merchant—or more commonly a “brusher” middleman who they pay—writes positive reviews using the names of the recipients.

The goal is to appear higher in the search rankings of online platforms like Amazon. The algorithms of those platforms favor merchants with lots of sales and positive reviews from verified purchasers. Since the name on the review belongs to someone who did receive the product, it’s considered a verified purchase. Brushing can make it appear that hundreds or thousands of people have received and praised the merchant’s product.

When did brushing start?

One of the first uses of “brushing” in English-language media came via a 2015 Wall Street Journal report. The report is titled “They call it brushing” and describes how brushing had become a problem in China where unscrupulous merchants used the technique to game Alibabab, the country’s giant ecommerce platform. And in 2016, the Financial Times reported “China’s ecommerce sites try to sweep away brushing.”

The brushing phenomenon appears to have come to the U.S. not longer after that. A 2018 article by an organization called Scam Busters warned that Chinese brushers had begun large scale campaigns that targeted U.S. consumers.

Is brushing legal?

Probably not. It is certainly a violation of the terms of service of websites like Amazon, and several reports say the tactic violates laws against false advertising as well.

Who are the brushers behind the mystery seeds?

That remains unclear. The FBI and USDA have yet to suggest who is behind the recent seed campaign. Likewise, it is unclear if the brushers behind the campaign are selling seeds or gardening supplies—or if the seeds just represent an inexpensive item to put in the mail as part of the scam.

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This story was originally featured on Fortune.com