The classic example of the Confounder effect is the strong association between ice cream sales and crime rate.

The observed relationship is that as ice cream sales increase, the crime rate increases, however we attribute the increase in ice cream sales and crime rate both to change in season from winter to spring and summer.

During nice weather, there are more ice cream sales, but also more people outside in close proximity to each other, increasing the potential for crime.

If we hold the weather constant, we will observe that ice cream sales have no effect on the crime rate.