All answers thus far are somewhat inaccurate or vague. A more detailed answer:
Apple's touchscreen devices and trackpads use capacitive touchscreen sensing. This technology measures capacitance on a grid of points. When a capacitance within a certain threshold of human skin is met, the software reacts to the touch input detected in those points on the screen.
This is opposed to, for example, resistive touch screens, which are generally used in older or lower-end electronics. These have a slight squishy feel on the surface, and are much less responsive than capacitive screens, but work with any object as a stylus. This works by having two films of conductive grid lines. When an object causes a depression in the surface which pushes the two layers together, the point of contact can be measured by comparing the resistance of the various grid's lines.
Having made this distinction, any object used to interact with a capacitive touch panel must have a similar capacitance to that of skin. Things such as meat would work (sausages were suggested), but aren't very practical. However, styluses made to work specifically with these screens can easily be found online. Those might be the perfect solution for you, since they're like a long thin pen used for the touch screen that doesn't obscure your view as much as fat thumbs might.