A sentence is like...

To me a sentence is almost like a cake.  It starts out with random ideas, and words, and meaning; like a cake starts out with many ingredients that add certain textures or tastes to the cake.  The sentence/cake starts out in a raw, unpolished form.  But once all of the ingredients or ideas have been brought together and baked, or put together in the form of a sentence you get a wonderfully delicious end product.