Counter target activated ability from an artifact source.
"How dull it is to pause, to make an end,/ To rust unburnished, not to shine in use,/ As though to breathe were life!" —Alfred, Lord Tennyson, "Ulysses"
2004-10-04
Can't be used on mana abilities because they don't use the stack. They've already resolved by the time you get priority.
2016-06-08
Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keywords are activated abilities and will have colons in their reminder text.