When are you safe outside if you hear thunder?

When there is thunder, then there would be lightning. The thunder and lightning are just reactions in the atmosphere involving electrons in the water molecules. Its product is the lightning and thunder, but lightning comes first because light is faster than sound. So, when you are outside, you can't really outrun a lightning because it is the fastest matter. However, you can avoid being hit if you don't stay near to isolated trees, electrical posts and metals. Remember that lightning is just a stream of electrons so they could easily conduct with metals. Remove any metals from your body and stay low.