Two of the ten knives have been lost, and there are eight left because this question is a puzzle, and the answer is not the conventional mathematical subtraction. There used to be ten knives, but then two knives were lost. The rest is not ten MINUS two equals eight, but the original ten knives.

The key to this problem is to understand the meaning of the action of "throwing". The loss of two knives means that there is no problem with these two knives, and the rest are the original ten knives. So according to this logic, after losing two of the ten knives, there are eight left.