... [M]athematician [is] a device for converting coffee into formulas.
attributed to Albert Einstein[see the update below]
The attribution looks suspicious, and I have not been able to confirm it. [Update: This appears to be a variant of Alfréd Rényi's original: "a mathematician is a machine for converting coffee into theorems." Thanks to Rahul for his comment and the link.] In any case, it's a great way to start the morning!