For eight-year-old Toby, who is deaf, watching films or TV on streaming platforms can sometimes be a bit pointless - because so many of them don't have sign language versions. We have captions but they don't really do anything for him because it goes quite fast. He would just watch and not get much from it," his dad Jarod Mills [said]. But now, Toby has some help thanks to an app developed by a 17-year-old A-level student. Mariella Satow, who has dual UK-US citizenship, lives in the UK but has been stuck in New York since summer 2020 because of Covid travel restrictions. In that do-something-new phase of lockdown, Mariella created a signing app called SignUp. She got the idea when she was teaching herself American Sign Language (ASL) - one of hundreds of sign languages used across the world. Mariella wanted to watch TV shows to help her learn, so was disappointed to discover how few had signed versions. It's taken a year for Mariella to develop the technology, with lots of help from ASL teachers and the deaf community. The app is available in the US as a Google Chrome extension - with an interpreter appearing in a box once the film starts playing. It only works on Disney Plus films at the moment, because that's where Mariella thought she could help the most children. Jarod, who works in Kentucky at a school for deaf children, says it was "exciting" watching Toby use Mariella's invention. "The app creates a level playing field," he says. "Kids are getting that understanding and information like any hearing child does."