None of the others have anything to say to that.
"Well, that's... a rather more gruesome tale than I had expected but it illustrates my point well, I think." Tolfdir turns to face all students. "You all know the very basics of control; feeling your magicka within you and shaping it into something that can do what you want it to do. But there's more to it than just that. Waste magicka can ignite; slight errors in shaping it can create wildly different effects, especially as spells get more advanced and complex; hostile disruption, or even a momentary distraction, can have catastrophic effects if you do not have enough mastery to make sure the spell merely fizzles out safely.
"Today's lecture with Master Faralda will touch upon this subject, and I highly recommend attending it. But until then, I would have you practise control. Does anyone here not know the basic flame spell?" Headshakes all around. "Excellent. Then this will be your exercise: start a flame," and here's a flame above Tolfdir's palm, "and change its colour." The flame goes from orange to red to blue to white to purple and back to orange. "There are two basic ways you can do it: with direct shaping of the effect, or with illusions. The latter is much more complex than the former, but also requires much more fine control, so I will accept either approach.
"Do not sit down while doing this; it will be easier to heal you as necessary if you do not accidentally set your clothes on fire, and it helps with focus." Tolfdir turns around and concentrates on a complex-looking spell that involves a lot of gesturing, and then a table appears in front of him with jugs full of water and empty cups. "But do take breaks as needed, and stay hydrated. This is not a test of endurance, and if you falter because you did not take a break when you needed to that is an indication that you need to work on your awareness."