Nuclear terrorism is one of the most serious threats of our time. Even one such attack could inflict mass casualties and create immense suffering and unwanted change in the world forever. This prospect should compel all of us to act to prevent such a catastrophe.

I call on men and boys everywhere to join us. Violence against women and girls will not be eradicated until all of us - men and boys - refuse to tolerate it.

We must unite. Violence against women cannot be tolerated, in any form, in any context, in any circumstance, by any political leader or by any government.

Climate change...is, simply, the greatest collective challenge we face as a human family.

I spend most of my time in South Korea but at least one-third of my time is spent traveling around the world, meeting people, speaking about climate change and sustainable development and trying to foster global citizenship especially for young people. What I observe is there clearly is a need for global citizenship. That is exactly right. We are having troubles around the world. We need global vision.

You are here to step into the shoes of UN ambassadors - to draft resolutions, to plot strategy, to negotiate with your allies as well as your adversaries. Your goal may be to resolve a conflict, to cope with a natural disaster or to bring nations together on an issue like climate change. You may be playing a role, but you are also preparing for life. You are acting as global citizens.

With her courage and determination, Malala has shown what terrorists fear most: a girl with a book.