We considered a button but ended up going against it. It doesn't scale - the button doesn't give us enough data. People don't time their presses well enough and if the app becomes successful we'll have a hard time telling apart presses that occur close to each other (both in time and space).

We've also considered a 4 digit pin but that confused users. It turns out that users intuitively understand the shake UX to transfer the contents from one phone to another.