1. 如果 hash 需要初值的話，可以使用 `Hash.new(default_value)`
```ruby
usually_brown = Hash.new("brown")
pretending_to_be_there = usually_brown[:zig]
puts "Pretending to be there:"
p pretending_to_be_there

# => Pretending to be there:
# => "brown"
```

2. `1.next == 2`
3. method 也是一個物件
```ruby
'aaa'.method('upcase') # => String#upcase()
'aaa'.method('upcase').call # => "AAA"
'aaa'.upcase # => "AAA"
```
4. deep_symbolize_keys
```ruby
hash = { 'person' => { 'name' => 'Rob', 'age' => '28' } }

hash.deep_symbolize_keys
# => { person: { name: "Rob", age: "28" } }
```