is it possible to make an internal hard drive into an external harddrive?

i tried pluging in an old hard drive to my new pc but the things wouldnt fit into each other, what can i do ?

External hard drives are just internals hard drives fitted into an external case.

If the ATA/IDE cable plug didn't fit the hard drive then it may be a Slim drive (Laptop) a SATA drive or SCSI.

Details here:

http://www.computerhope.com/help/ide.htm

The drive should have a label on it identifying the type.