append vs concat

append.rb

string = 'hi'
# => "hi"

string.object_id
# => 67890

string.object_id # yup, same every time
# => 67890

string << ' you'
# => "hi you"

string.object_id
# => 67890

## ^ Look, same object_id, so same Object as the original String with #<<

string = 'hi'
# => "hi"

string.object_id
# => 40572

string.object_id # still same every time
# => 40572

string += ' you'
# => "hi you"

string.object_id # OMGWTFBBQ
# => 12345

## ^ Brand new Object with #+=