Halo Fountain is a newly spoiled card for New Capenna:
Now say you turn Halo Fountain into a creature (there's a variety of methods to do this, say Karn, Silver Golem for example).
Say you wanted to activate its first ability. You pay W, tap it, then untap it. Does that work?
Alternatively, since you can pay costs in any order, can you pay W, untap it, then tap it?