It is the minimum required to complete a circuit. You need a live and neutral.
A two pronged plug does not have a saftey Earth connection. The device operated must be double insulated and have no metal parts visible, that could become live.
why are metal prongs of a plug sealed in plastic and rubber
A plug with three prongs.
A fork has prongs at the end of it. (Actually a fork has tines. A plug has prongs.)
A wall socket (either switched or unswitched) is the place into which the prongs of a plug is plugged in.
My heater does not have 2 prongs.
All relays are replaceable like fuses, you just pull it out and plug in a new on. It will have 2 or 3 prongs like a plug.
to stop them from moving
Plastic insulates the prongs from one another, yet is hard enough to keep them at the proper distance to be put into the wall outlet and allow a person to grip the plug.
You can't buy an adapter but you can buy either the receptical or the plug and change one out to match the other.
The ground prong (usually of a male plug) is a metal tab or rod connected to the conductor which is to be grounded.
You mount it on the wall close to a plug in
One is live, one neutral, and one earthed.