Can't get my tank to shoot right

[SteveCRM]

I can't figure this out:

Public Sub CheckKeys()
'SHOOTING
If KD(vbKeySpace) = True Then
Shoot
End If
End Sub

Public Sub Shoot()
Shot.y = Shot.y - 1
End Sub

When I hold down the space bar, the bullet goes; but when i lift it, the bullet stops. How do I keep the function going?