You have to find some other way to tap it, such as paying a waterbend cost or attacking
Curious Herd murders artifacts matter by giving you a Beast for each artifact that player controls
There was always an asterisk at the end of kill target creature, whether it was a restriction on what you could kill, not being able to always kill what you wanted to kill, or a bunch of crap that happened after you killed it
This is an example of what happens when you have cheap extra turn spells, and Time Walking your opponent (or yourself) is still something referred to in Magic slang
Then at a later point in the game, you can tap Ice Cauldron to use the mana you've invested and place the spell on the stack
Each of these will reward you with some number of packs from a different Magic: The Gathering set - and, due to how expensive the cards can be even when you're only playing digitally, they're effectively essential to every player both old and new