Why do you use (&block)

You are right, Anthony. If you try this in an irb shell or put it in a .rb file and call it from the CLI:

def foo(n, &block) yield end

foo (1) { |n| n + 1 }

...you will get a NoMethodError: undefined method `+' for nil:NilClass

But if you change foo to this:

def foo(n, &block) block.call(n) end

or this:

def foo(n, &block) yield n end

...in either form the program will return 2.

After reading your reply, I looked at your original question again and I think on my first attempt I may have misunderstood what you were trying to find out. It seems you have a good grasp of how to use block.call and yield -- maybe you were just wondering why the instructor used &block for passing the block. You can actually put anything you want in that argument placeholder: &block, &clock, &glock, &bar...as long as it begins with an ampersand (&) and follows the ruby variable name snake-case convention (lowercase word or words separated by an underscore "_"). But in all the examples I've ever seen in several Ruby books, the convention has always been to use the term &block because it makes it clear to anyone reading the code that a block is expected. I think the instructor is just demonstrating different options for passing blocks to methods. As I learn other languages like Javascript and Python, I'm finding that developers/engineers who come to Ruby from another language get frustrated by how many different ways you can do something in Ruby.

To be honest, I haven't done that particular course module yet, so I haven't seen the entire code of the hide example. But I've been programming in Ruby for about 9 months before I started using Treehouse, and while I'm no expert, I seek out opportunities to try to help others starting out with Ruby. I hope my explanation can be applied to the code in your specific module.