Definitely the toughest interview I've had in my entire career. The phone interview was relaxed and the interviewer (HR) only asked a few questions regarding my background and what experiences I had in my other companies. HR then sent my profile to the hiring team which approved for me to fly to Cupertino for a Group interview. Most of the people were relaxed and asked common sense questions to what the job required and my skill/experience and how it related to the position. But then there were a couple of people that clearly wanted to make the interview very very difficult by asking theoretical questions and having me answer by using dynamic and kinematics formulas. Now in the real world if any of these equations are needed you would just consult your old engineering book. It would be unpractical to have an engineer remember from memory every single equation he/she learned during their Undergrad. In the end, what I got from the interview was that this company looks for people who are more academically inclined (book smart) first and foremost than someone who has experience or is practical at problem solving.