Re 2: This works because :: is used for scope resolution in Ruby: https://en.wikipedia.org/wiki/Scope_resolution_operator#Ruby

Re 6: The special variables become a lot less magic if you require 'English: https://ruby-doc.org/stdlib-2.3.3/libdoc/English/rdoc/English.html

Re 8: I mainly use them in code golf 😉 Or when I want to be really explicit about expecting a one character string.

Re 13 and 14: both the BEGIN and END block as well as the flip-flop operator are remnants of Ruby’s Perl legacy. Though it’s been a long time since I actually saw them in code.

Now for some of my personal favorites:

*) Inheritance: the right-hand side of < can be any valid Ruby expression as long as it resolves to a class.

class IdentityCrisis < [Array, Hash].sample ; end

*) Unary minus: to define a unary minus operation, you have to use -@.

Also if you wanna confuse your friends, make them guess what this does: (1..5).map(&:-@) 😉

Infinity:

Before Float::INFINITY was added in 1.9 we used to make our own: Inf = 1 / 0.0 #=> Infinity

Destructuring addendum: fairly obvious, but restructuring works for assignments as well: