TAMPA, Fla.—Another holiday season, another online scam.

The Better Business Bureau is warning people about an online “secret sister” gift exchange that sounds fun but is usually too good to be true.

A popular post started circulating on social media in 2015. It tells people they will get up to 36 gifts in exchange for sending one gift of a $10 value, the BBB said.

The BBB said the problem is gift chains like this aren’t really gift chains, but pyramid schemes.

Here is how this scheme works:

If one person gets one gift for a stranger, they could get as many as 36 gifts in return. This type of gift exchange may seem reasonable enough in theory: six friends invite six more friends, who all send gifts to the participant in spot one before that person’s name is removed. This process repeats itself with the participant in the two spot, and so on, the Better Business Bureau explains on its website.

Of course, starting this gift exchange comes with a catch: people will need to disclose their personal information, like an address.

Don’t forget, pyramid schemes are illegal if money or other items of value are requested with the assurance of a sizable return for those who participate, the BBB warned.

The BBB says if people get chain letters by mail, email or social media, especially one that involves money or gifts they should ignore it.

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