why does Emilia take Desdemonas handcheif and tell her husband that she has it?

I think Emilia always meant to give the handkerchief back to Desdemona. Emilia knew hoe precious it was to her. She simply saw it and picked it up. She mentions that she intended to have a copy made and give it to her husband. She doesn't have time to do this. Emilia sees Iago and says , "I have a thing for you" . She knows her husband wants it but not the reason.