During a thunderstorm, we also see lightning.  If lightning comes into contact with an object that can conduct electricity, it will do so.  If you are touching that object, then that electricity that discharges from the lightning strike will be transferred to you.  The amount of charge that is in lightning is big enough to interfere with and actually stop your heart from beating. 

This is true not only for metal objects but also for water.  Completely pure water does not have a sufficient number of ions to act as charge carriers but tap water (even filtered tap water) still has a significant number of ions in it and will be able to conduct electricity.  Since we don't know exactly when or where lightning will strike, it's best to be in a safe place when we hear thunder since lightning is typically followed by thunder.