Severus' reaction was quick. His wand was in his hand in an instant and he cast a shielding spell around the glass and water, containing it within its own bubble. He guided the bubble to the sink, lowering it before releasing the shield and letting the contents of the bubble splash against the basin. His actions were quick and wordless, more instinct than anything else.
"Practice sounds like a good idea," he said quirking an eyebrow up at her. "Actions which require a lot of control or preciseness are much more difficult wandless than spells which have a general or larger are of effect. It's actually easier to cast, I don't know, locomotor mortis on a person than it is freeze water in a glass."
He pushed a strand of hair behind his ear before getting some more take away from the cartons. Even though he was almost too skinny, Severus could eat his weight in food without gaining weight. He almost never did, though, preferring to maintain control over what he ate. It was a sign of his comfort with Lily that he took another serving.
"Spells that you perform regularly wordless and wandless will start to come more naturally that way. Protego, legilimens, and occlumens are three of the easiest for me." Severus averted his eyes at that moment, realising that confession revealed more clearly precisely the type of thing he was doing on a regular basis. Dueling, reading minds, and shielding his own. It was all a juggle to stay alive; he wasn't sure Lily would understand that.