You do not really know that you have a hand. Here is why. Imagine the possibility in which you are only a handless brain floating in a vat, wired to a super computer that sends to your brain signals that perfectly simulate ordinary sensory experiences. Now, if you really know that you have a hand, you must be in a position to know that you are not such a handless brain in a vat. But are you in a position to know that

If you do not know you are a brain in a vat, you do not have hands.

Explanation:

This premise is plausible as to understand the handless brain which is floating in a vat. The brain is connected to a super computer and it transmits messages to the brain. The brain receives those message and give signals to perform tasks.

answer; i believe the answer you're looking for is (nanotechnology);

f7 should open the spelling and grammer dialog box