Order of Paying costs for Activating abilities

[Crazy Monkey]

This question is specifically related to the new card

SPOILER

Show

Hide

Halo Fountain, and specifically if it is a creature such as after a Sydri, Galvanic Genius or Karn, Silver Golem activation

but the can be generalized.

If an activated ability cost of a creature was ", untap a tapped creature:" could that cost be paid by the tapping, then untapping the same object as part of paying the cost? 602.2 indicates that costs must be paid in order, which I understand to mean that by the time the second half of the cost needs to be paid, the object would be tapped.

[WizardMN]

Paying costs for activating abilities follow the same rules as casting spells. Namely:

601.2h. The player pays the total cost. First, they pay all costs that don't involve random elements or moving objects from the library to a public zone, in any order. Then they pay all remaining costs in any order. Partial payments are not allowed. Unpayable costs can't be paid.

So you can pay them in any order. If you tap the Halo as part of the cost, and it is a creature somehow, you can untap it as the third part of paying the cost if you want.