Press the button on its crown and the face in the window turns to the next mood.
A desk egg with 5 faces printed round a drum inside. There is no battery and no motor: your thumb is the whole drive, working the same click-and-turn cam you know from a retractable pen.
At rest the core sits with one face centred in the window. The other four wait out of sight round the back of the drum.
Nothing else drives this egg. Press the button on the crown and it slides straight down. It is not free to turn, so all it can do is push.
Under the shell, the button's sawtooth tip meets a rotor with 5 matching teeth. The rotor drops out of the fixed collar, squeezes the spring and is shoved half a tooth round.
Let go and the spring lifts the rotor back into the collar. The collar's ramps turn it the other half tooth, 72° in all, and it seats there.
The rotor slides on 5 keys inside the drum, so the drum turns with it, and the next face lands in the window.
Surprised, sleepy, sad, angry. Press number 5 brings happy back, and the cycle starts over.
Each press turns the core 72°, one fifth of a circle, so every face comes round in order and none can be skipped.
The shell twists off the base. That is how the egg goes together, and how the face core comes out. From the top down:
Set it on the desk and let the face say how the day is going.