How can I break it to her

So my daughter is 10 and her father hasn't been in the picture since she was 6 months old cause he was deported. I got with someone else when she was a baby and he is all she's ever known and calls him dad. But for a while now off and on she keeps asking me why her last name is different then mine and her sisters. I don't know how to tell her and I'm so afraid of how she'll take it. Anyone been thru this advice please cause she's not dumb and she'll eventually figure it out and I wanna tell her before that happens