- Create a function `createCircle`: make it fire on every click on the page, and create a `div` at the position of the mouse on the screen, setting its `background` to `white` and its class to `circle`
- Create a function `moveCircle`: make it fire when the mouse moves, and get the last circle created and makes it move along with the mouse
- Set a box in the center of the page ; when a circle is inside that box, it has to be purple ; once a circle enters the box, it is trapped inside and cannot go out of it anymore.