Why are bees yellow and black?

Answer 1

Not all bees are black and yellow however, colours such as black, red, yellow and orange are used a lot in the animal kingdom as warnings that the animal has a bad taste or poision. Humans also use these colours in road signs and safety labels. Some of the chemicals which cause the colours are derived from urine (see linked "Nature" article if your are a sciencey type). The yellow and black banding is shared with the wasp and both species live in the same environment. This would confer added protection to both species as a predator learning not to eat one species would stay away for the other due to their similarity. From an evolutionary point of view the colouring is not helpful to the individual as it makes them easy to spot and so easy to eat however, once a predator has eaten one bee it is unlikley to try another and so the rest of the hive are saved from being eaten.

On the light side:

My mom said that bees are black and yellow just because they are, and they want to be and we have no say in that.

There really isn't a better answer. They are whatever colour they are.

Answer 2

In nature, anything with bold yellow and black stripes is usually poisonous, unpalatable or dangerous. Wasps and bees fall into the last category because they can sting. Some insects, such as hoverflies, imitate the colouring as a form of self-protection, even though they are otherwise harmless.