I do drink coffee/water while driving, however a few points due come to mind... The most important is common sense...
When you raise the glass/cup to your lips to drink, unless it is completely full, your vision will be blocked, you will tilt your head back to drink and your eyes will leave the road...
Same type of situation to locate your glass or replace it on console.
The same points can be placed on dealing with cigarettes, phones, radio, spilling your drink, dropping something, talking to your passenger, etc...
In truth, most including myself, cannot truly mentally multi-task... You are either concentrating on your driving or your phone conversation, it doesn't matter if you are holding it in your hand or not...
The first ones that say they can have never had someone outside of their vehicle video them... Everyone believes they can though...
I have proved this to several people that were positive they were a safe driver and could talk while driving, until they saw themselves how others see them...
Similar to your eyes, you can only read exactly what you are looking at, you have peripheral vision, however you can't read with it...
Most can only concentrate on one thing at a time. Sure you can do a quick small talk, just as you can do a quick glance...
Taking your eyes off of the road/not scanning, even for a second, is dangerous while merging into traffic, making turns, entering intersections, heavy traffic, weather, construction/school zones, etc...