In order to charge the glass rod, why do we RUB it with a sillk cloth?
If you left a glass rod wrapped in a silk cloth for "some time" there would be a net transfer of charge.

The difference rubbing makes is accelerates the process due to charge transfer aided by friction

Look up the Triboelectric effect



Materials close together on the series may not experience any charge transfer if simply touched (add friction into it however...) while materials further apart will.

What this also means is two materials that were initially GROUNDED that are then placed in contact with each other &amp; then ungrounded will experience a charge transfer over time.

The rule of thumb is the better the insulator, the great the affinity for charge (there are obviously exceptions - see the chart). Walking on a carpet will generate 35kV of static, sliding down a fibreglass slide will generate 50kV etc...