when u can be online ?

I asked my son. He said - Use you brain- you know? like Hercule Poirot

It means to use your brains and intellect, as in ' If you'd only use your grey matter, you'd see the answer in a minute.' This expression refers to actual brain tissue that is grey in color. Agatha Christie's fictional detective, Hercule Poirot, constantly alluded to 'using the little grey cells' for solving a crime. Nowadays retirees are encouraged to keep using their brains by doing crosswords, sudoku, computer research etc to avoid getting Alzheimer's disease.

You can refer to your intelligence or your brains as grey matter.