lungs are used for breathing air by birds & breathing is a kind of function ->  lungs perform the function of breathing in birds  skin is used for breathing air by frogs & breathing is a kind of function ->  skin performs the function of breathing in frogs  lungs perform the function of breathing in birds & skin performs the function of breathing in frogs ->  lungs in birds and skin on frogs both perform the function of breathing  lungs in birds and skin on frogs both perform the function of breathing & if two things perform the same function then those two things are similar -> the function of skin on frogs is similar to the function of lungs in birds