It's simple enough to use. Pose in the middle of your camera's field of view and wait a few moments for the system to try and read your expression. One game invites you to trick the system using fake smiles, while the other asks you to challenge the system to recognise the difference between a wink and a blink.
It's a fun toy, but Dovetail Labs has designed it as a conversation-starter to examine the ethics of emotion recognition technology and the wider implications of its mass adoption. Such systems often have a racial bias, and these browser games come with a note explaining that the system's effectiveness will vary if you're not a white male. Naturally such biases may have serious consequences in the fields of, say, law enforcement.
And of course there are privacy issues. The site says it doesn't store any data, and it invites readers to submit thoughts on the technology anonymously. We can't vouch for this, so use at your own risk.
Emojify is an amusing way to explore a serious topic, but I couldn't resist running some famous faces through the system to see if can glean their innermost feelings.