Your Kitchen Sink:

The faucets on your kitchen sink are standard plumbing fixtures.  Since they are designed to protect you automatically, you don't need to do anything to make them safe.  The manufacturer does this by providing an air gap between the end of the faucet and the overflow water line of the sink.  A typical unsafe condition occurs when a hose on the end of the faucet is submerged into dirty water in the sink.  The dirty water can be drawn back into the safe water supply.
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Outdoor Faucets:

The ordinary garden hose is the most common way to contaminate the water supply! This can happen when one end of the garden hose is attached to an outdoor faucet (sill cock), and the other end of the hose connects to an aspirator type bottle.  Insecticides or other chemicals in the aspirator bottle can be siphoned back into the drinking water supply.  Or when filling a livestock watering trough and letting the garden hose just lay in the trough unattended.  You can easily prevent the possibility of this type of contamination by ensuring there is an "air gap" between the watering trough and the garden hose, or by installing a hose bib vacuum breaker.  This is a small, inexpensive device you can get at most hardware stores and simply attaches to a threaded water faucet.