Explain how too much current passing through a conductor, may cause damage to appliances and people

background info...an intro into electrical safety

Too much current through a conductor generates heat faster than the conductor can dissipate it.
So the conductor gets hotter and hotter and hotter...
Soon the insulation around the conductor starts to smoulder, creating a smoke hazard, and a small fire.
The exposed bare conductor then presents a hazard of electric shock to anybody who touches it later.
If the excessive current lasts long enough, the temperature around the conductor will exceed the combustion temperature of the nearby materials inside the appliance, and a fire will start.
The fire inside the appliance will then spread to the surroundings creating a major fire hazard, possibly burning down the entire building.

[Of course a fuse or circuit breaker should have blown first!]

A conductor has resistance. The smaller the conductor, the larger the resistance. When a current passes through this resistance it causes heat. The higher the current, and the higher the resistance, the higher the heat generated ( the equation is Heat (power) = Current squared times resistance).

If the current is too high (relative to the size of the wire), the heat generated can be enough to melt the copper or to start a fire.