Apple has introduced much-anticipated iPhone X a few weeks ago. The new phone comes with a fresh design, a variety of features including the facial recognition system. However, tech giant says iPhone X Face ID is not for everyone. The system may be less accurate and therefore less secure for kids and twins.

See also: Apple’s iPhone X Comes With 3GB RAM And 2,716 mAh Battery

The tenth anniversary iPhone X doesn’t have a home button, so there is no Touch ID any longer. Instead, people will be able to use Face ID to unlock their phones just by looking at the device.

Apple has provided more details on iPhone X Face ID. The company released a security guide on Wednesday about the facial recognition system.

Facial ID is much more secure than Touch ID. With Touch ID, chances that a random person unlocks your phone with their fingerprint are 1 in a 50000. When it comes to Face ID a probability that someone else’s face unlocks your phone is one in a million.

But, the company says iPhone X Face ID is not for everyone – it may be less accurate for kids and twins. The company suggests that children should rely on a passcode and use that method to unlock their phones. If you have a twin or a sibling that looks like you, you may want to use passcode as well.

“The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you’re concerned about this, we recommend using a passcode to authenticate.”, Apple said in the security guide.

The iPhone X is the most expensive iPhone ever. Its price starts at $999 for 64 GB model. The preorders will start on October 27 and the phone will be in stores November 3.