Question

why does it happen fruits can easily be cut with a sharp knife

Answer 1

because the area of edge on which the pressure is applied is quite less which in turn gives rise to a greater effect and as a result the fruit cuts

Answer 2

because pressure = force / area
which means that pressure is inversely proportional to area
that implies if area is less pressure will be more
similar is in the case of sharp knife
area is less that increases the pressure n fruits can be easily cut