A closure captures all variables from its environment that it is using (no more than that). They can be captured by reference (which is the default), or by-move (which is done with the `move` keyword). In case it is being captured by move, _all_ captured variables will be moved. If you wish to capture a specific variable by reference in a move closure, create a reference (`let y = &x`) outside of the closure and use the reference y instead of x inside.