def functions and floats

[buran]

Just to add to what killerrex already said - you need to supply some number as argument c when calling function, e.g.
print celsius(10.0), i.e. you want to convert 10.0 degrees cecsius

also, more appropriate name of the function is fahrenheit (or something like celsius2farenheit) because you convert Celsius to farenheit, not the other way around

also instead of one char argument c, better use something descriptive like celcius