How do you use soul converter?

[amar7]

How do you use soul converter?

[MobiusPizza]

http://bindingofisaacrebirth.gamepedia.com/Converter

[Nazemec]

You mean Guppy's Paw (pickup quote: "soul converter")? Its effect is to transform one of your red hearts into 3 soul hearts. Which means it won't do anything if you don't have any red hearts.

However, if you mean the Converter, its effect is to transform two of your soul/black hearts into one red heart. Again, it won't do anything if you don't have enough soul/black hearts.

Detailed wiki pages:
http://bindingofisaacrebirth.gamepedia.com/Guppy%27s_Paw
http://bindingofisaacrebirth.gamepedia.com/Converter

Edit: half of my post, ruined by a pizza... :c

[amar7]

I mean guppy's paw, what key do I press to use it?

[Nazemec]

Same for every active item, default is spacebar.