Another difference between CD-ROMs and disk drives is the extra error checking that is included on CD-ROMs due to the fact that their surfaces are directly exposed to humans and other dangerous things in the environment, whereas disk drives are sealed. Even disk drives include some error detecting bits, such as simple parity, to discover if a byte has suffered damage. But CD-ROMs, which can't be fixed by rewriting, include error correction bits which allow the computer to reproduce the correct bit pattern, as long as not too many bits have been damaged. It is possible, for example, to take a knife and scratch the CD-ROM without impairing its data, as long as the scratch isn't too wide and as long as it goes radially, i.e. from the center to the edge. A scratch that is angular may make the data unrecoverable.