This is critical because it eliminates early zealots, zerglings, cannons that players tend to pull on you.
If you suspect void rays, explore the map.
Example here
In one match you can use up to 30 runes (at level 30) but you can also change the runes after the match if you want to play another champion.
Which brings me back to where I started this article: Just what do you think you're doing, Dave?