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Hashes are key-value collections. Keys can be anything (most often symbols or strings).

ages = { "Alice" => 30, "Bob" => 25 }   # rocket form (any key type)
ages = { alice: 30, bob: 25 }              # symbol-key shorthand

puts ages[:alice]      # 30
ages[:carol] = 40       # add
ages.delete(:bob)
puts ages.size           # 2

ages.each do |name, age|
    puts "#{name}: #{age}"
end

Missing keys return nil by default. To set a different default: Hash.new(0) for a counter:

counts = Hash.new(0)
words.each { |w| counts[w] += 1 }   # safe even for unseen words

Hash.new { |h, k| h[k] = [] } lets you compute the default on demand — very handy for grouping.