According to the causal theory of knowledge, I only know p if my belief in p, or my evidence for that belief, is caused by the worldly state of affairs that p reports. Thus, I only know that I won the lottery if my belief that I won the lottery, or my evidence for that belief, is caused by my lottery win.
If the causal theory of knowledge is true, then I don't know that I now have two hands. For even though I can see my hands, my evidence of my having two hands and my belief in my presently having two hands isn't caused by my present having of two hands. Rather, it is caused by my very near past having of two hands. Information travels from my hands to my brain at most at the speed of light, after all.
But I know that I now have two hands. Hence the causal theory of knowledge is false.
The argument extends to any case of our knowledge that some external physical event is now occurring.
Objection 1: A temporally extended present event can be causally observed, because its past parts can cause us to perceive it.
Response: If E is such an event, then the causal theory of knowledge may allow us to know that E occurs tenselessly, but not that E is now occurring.
Objection 2: In ordinary language, "now" refers to a temporally extended period of time that reaches somewhat back. So the very close past existence of my hands is enough to make "I now have two hands" true.
Response: If it turned out—as in some world it does—that my hands ceased to exist in the interval between the light bouncing off the hands and my forming the belief that I have hands, we would say that my belief was mistaken. Hence the content of my belief that "I now have two hands" requires the literal present existence of two hands, not just a very close past one.