Why does an object float or sink when placed on the surface of water?

Explanation

When an object is put in water, then two forces act on it:
(i) weight of the object acting downwards (which tends to pull down the object), and
(ii) buoyant force (or upthrust) acting upwards (which tends to push up the object)
Now, whether an object will float or sink in water will depend on the relative magnitudes of these two forces (weight and buoyant force) acting on the object in opposite directions.
If the buoyant force (or upthrust) exerted by water is equal to or greater than the weight of the object, will float in water.
If the buoyant force (or upthrust) exerted by water is less than the weight of the object,will sink.