We believe in heaven because of water.

We have to drink water to stay alive.

It is vital to our existance and so we go to great lenghts to get it.

We (Primitave man) would carry it long distances from lakes and rivers back to our villages.
And we know therefore, how damn heavy it is. We also know that it falls down from the sky and fills all the lakes and rivers.

So knowing how heavy it is and that is is stored up in the sky we can deduce that there must be something up there capable of taking all that weight.

Obviously we (primitave man) know nothing about clouds and water vapor.

Voila! The heaven above us story.