Stress/Strain Graph Question

[SDPHD]

I was wondering what the reason was for the curve of a material, in both a stress/strain graph and force/change in length graph, reaching a high point and coming back down before breaking (instead of just breaking at the highest point).

Like this -



Thanks.

[yawho]

i think i know why but don't know how to explain it
my teacher asked us to try it and experience it