## Let me tell you a story from my teaching

On Wednesday I was teaching an exercise class on graph theory. There was this one exercise that was troubling me for a couple of days, I couldn't solve it and it was frustrating.

## Start to jet lag behind. Moving to fairer competitions.

It is safe to say that traveling impacts the peak performance of teams and athletes in general - studies have been done across all kinds of sports that confirm this intuitive idea. Thus, to avoid unfair- and unhappiness, an organizer should aim to minimize the effect of travel time disparities.

## From Tweets to Communication Networks

Network visualizations have the power to display how we communicate with each other in social media. We can simply depict message exchanges using communication networks. In such a network, nodes represent users, and there exists an edge between two nodes when the corresponding users exchange information (an email or a tweet).

## Eigenvalues to the rescue

On a quiet afternoon, professor Meth is working in her office in Leiden on some tantalizing mathematics problems. Suddenly, someone knocking on her door nervously disrupts the silence.

## What have Donald Trump, an epidemiologist and a rock in common?

In this article, dear reader, I am going to show you in which way the development of your opinion during the last political issue, the spread of a virus among your acquaintances during the current pandemic, and the alignment of some particles lying inside the device from which you are reading this article are extremely comparable phenomena.

## The travelling salesman problem

Suppose you have a delivery service. You have one truck and have to deliver a large number of parcels to different cities in the country every day. Then you run into the following problem: in which order should you visit the cities?

## How do you decide who is the most important?

Imagine you’re in a remote village and only have a limited number of vaccines to distribute to protect the community from a deadly virus, who do you vaccinate?

A difficult decision, but necessary. Assuming that the disease is just as deadly for everyone in the community, the best way to prevent deaths is to contain the spread of the virus.

## How many colors do you need to color a map?

A mathematician once told me that problems that are very simple to state can be very deceiving, and sometimes turn out to be extremely difficult to solve. One such problem was the Four Color Problem.

## Netsweeper: playing Minesweeper without the old-fashioned grid

If you were born in the previous century, then chances are high that you have spent quite some hours playing Minesweeper, the classic puzzle game that used to be installed on every computer. In this article, we will present a new network-based version of this game and show how network science can help you play this game.

## Percolation theory: about math and gossip

Percolation theory is a branch of mathematics at the interface between probability theory and graph theory. The term 'percolation' originates from materials science. A representative question is as follows. Suppose some liquid is poured over a porous material. Will the liquid be able to make its way from hole to hole and reach the bottom?