module SiUnits
        def mega;  self * 1000.kilo;   end
        def kilo;  self * 1000;        end
        def milli; self * 0.001;       end
        def micro; self * 0.001.milli; end

        def seconds; self;              end
        def minutes; self * 60;         end
        def hours;   self * 60.minutes; end
        def days;    self * 24.hours;   end
        def years;   self * 365.days;   end

        def metres; self; end

        def grams; self * 0.001; end
end

class Float; include SiUnits; end
class Fixnum; include SiUnits; end

p 2.days             # => 172800
p 3.milli.metres     # => 0.003
p 4.kilo.grams       # => 4.0