Error: a nonstatic member reference must be relative to a specific object

[Computerjunkie]

I'm getting this error on this line:
critter.Play();

In the Critter.h file the Play() function is as such:
int critter:: Play()
{
system("cls");
cout << "You play with your critter making it happier!" << endl;
}

And i declare Play() in the public: like this:
int Play();

How would i get rid of this error? Thanks.