Take a standard length (about 3.5') of 1" tubular webbing, and tie an overhand or waterknot in the middle:
Insert one end of the webbing into the other ...
... about 10 - 12".
Pass the knot to the center of the inserted part and tighten (full weight, as for any tied sling!):
And VOILA! The COSMIC sling:
As Torontula Tami pointed out, this sling can also serve as a post-modern 'screamer', specially if you use really really strong cellotape or strips of "Hello Kitty" duct tape.
An added little bonus, here is a picture of the (quite useless) Klein sling, which has no inside and no outside: