bond - Why are some molecules unable to absorb infrared radiation?

Methane $\ce{CH4}$ is symmetrical and non polar, and so is oxygen $\ce{O2}$. But $\ce{O2}$ doesn't absorb IR. Why is that? People say that if a molecule can change its polarity then it will absorb IR, but I don't understand how a molecule changes its polarity while absorbing IR.

Please try to keep your answer simple — I don't have a strong background in infrared spectroscopy.