Why shouldn't you take antibiotics for a cold?

You may remember that there are broad categories of microorganisms that cause diseases, such as bacteria, viruses and fungi which all have different properties and ways of causing disease. Antibiotics are made to target bacteria specifically. However, the common cold is caused by viruses, which are different from bacteria. Therefore, if you have a common cold, taking antibiotics is not going to help you heal any quicker than if you just had some bed rest and lots of orange juice.
The real problem worsened by using antibiotics willy-nilly (like for a cold) is "antibiotic resistance", which is when new strains (sort of like breeds) of bacteria multiply. Imagine using an antibiotic that kills 99% of a harmful bacteria. The 1% left will have developed a way of not being beaten by that antibiotic. When that little 1% multiplies, it makes even more bacteria that will survive if you keep using the same antibiotic on it, and you end up with an antibiotic that works on 0% of that bacteria. Along with the fact that no-one has made a new class of antibiotic in more than 20 years, this is why antibiotic resistance is a huge conundrum for scientists in this day and age.