as with many 'lateral thinking' puzzles, there is more than one answer.
One solution, given the puzzle as described, is to take hold of one end of the string, put the other on the ground and twist it till the aforementioned 'pretzle' is near-formed, and only at that time take hold of the other end (through the pretzle loop) and pull.
There are others involving switching hold position (still within the 'rules'
, cutting the string in the middle etc etc etc.
However, I suspect the answer you are looking for is to cross your arms before picking up the string, then pull your arms uncrossed (english?) and bingo: one knotted string.
How many edits and i still can't spell...