Shell Programming and Scripting

I have three files that the string inside it I want to replace
so my code will be

#!/bin/bash
read -p "please input the old string:" string1
read -p "please input the new string:" string2
sed -i "s/string1/string2/g" *.c

but the problem is.. the string that I want to replace can't be changed
is that possible the code that i wrote is wrong ?
there is have any solution ?

thank you

string1 and string2 are variables. when you call variables, put $. something like this
eg

Code:

sed -i "s/$string1/$string2/g" *.c

Thank you very much