calculator.rb

puts "Hey! I'm a calculator!"

# Collects and checks for appropriate user input
print "I can add, subtract, divide, multiply, do exponents, and find a remainder. What would you like to do?  "
  operation = gets.chomp

possible_operations = ["+","add","addition","-","subtract","subtration","*","multiply","multiplication","/","divide","division","**","exponent","exponents","%","remainder","modulo"]
until possible_operations.include? operation
  print "\n\nWomp Womp. Like I said, I can add, subtract, divide, multiply, do exponents, and find a remainder. What would you like to do?  "
  operation = gets.chomp
end

print "Cool, cool. Which two numbers?\n"
  num1 = gets.chomp
  num2 = gets.chomp

if num1.include?(".") == true || num2.include?(".") == true
  num1 = num1.to_f
  num2 = num2.to_f
else
  num1 = num1.to_i
  num2 = num2.to_i

end


puts "\n\nMy work here is done:"
case operation
  when "+","add","addition"
    ans = num1 + num2
    puts "#{num1} + #{num2} = #{ans}"
  when "-","subtract","subtraction"
    ans = num1 - num2
    puts "#{num1} - #{num2} = #{ans}"
  when "*","multiply","multiplication"
    puts "#{num1} * #{num2} = #{num1 * num2}"
  when "/","divide","division"
    puts "#{num1} / #{num2} = #{num1 / num2}"
  when "**" , "exponents"
    puts "#{num1} ** #{num2} = #{num1 ** num2}"
  when "%", "remainder", "modulo"
    puts "#{num1} % #{num2} = #{num1 % num2}"
end

Simple and clearly done. How could you prevent incorrect input at the command line?