26

Looking for the LOST PAGE!

Search enginesThe most famous one is Google, have you heard of it? But it is not the only one: we also have Bing, Yahoo! and many others! When you are looking for informa-tion about something, like “dinosaurs”, you only need to write “dinosaurs” on the search bar, press “Enter” and you get a list of pages related to that word.

Are you looking for information about the dinosaurs? You can find almost everything on the internet! You only need to access the Web with your computer, smartphone or tablet to visit billions of different websites. Each one of these websites contains many different web pages, which contain texts, images, videos, videogames... But how can we find what we need inside this huge sea of web pages? We need search engines!

How do we sort out these pages? With mathematics! Search engines can identify the most and the less important results. The results you get when you write “dinosaurs” are always listed in this order. But what does it mean? In order to decide if a page is impor-tant or not, we need a lot of... mathematics! Search engines work thanks to two different ”branches” of mathematics: graph theory, which studies the struc-ture of the web, and numerical analysis, which studies how to “work out” the right calculations.

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A matter of linksWhen you are reading a web page, you should notice that some words are underlined and that if you click on them, you go to another page. This connection between two pages is called a “link”. If we represent a link with an arrow between the two pages, we have what mathematicians call a graph.

Graph of page A with a link to page B

Google works thanks to an algorithm (go to page 14 to find out what it is!) called PageRank, which works thanks to... links! PageRank is based on a simple but clever idea: if someone adds a link on his or her page to another one, it means that that page is important for that person. For example, if a famous scientist added a link to a web page on dinosaurs on his personal website, he probably thinks that that page is important! The results of a search are sorted according to this principle:

“A page is important if you can reach it through the links on other important pages”.

So, if many people added a link to the same web page on dinosaurs on their personal web page, the search engine would consider that web page more important than other ones. And if that web page has a link to another page, e.g. the website of a museum, that website will be considered important as well.

linkpage page

Even if a website is considered

important by many people, that

doesn’t mean that what is written

inside is true! All those people

could be wrong! So, do not

believe in everything you find on

the Internet: verify it first and

use your brain to understand if it

makes sense or not.

Pay attention!

28

The randomsurfer game!

Fold here

Cut out the surfer!

Rules:

1 All the players throw the dice. The player with the highest score can choose his/

her own website: you can chose either the red website (PLaNCK!’s website) or a blue one + a green one + a purple one. Then you continue clockwise. One by one, each player chooses his/her websites. You cannot choose sites that are already occupied by other players!

2 Write down the names of your websites.

3 Place the random surfer on PLaNCK!’s website. Throw the dice, starting from

the player who chose first, and then continue clockwise.

4 If the surfer comes to a crossroads, follow the arrow with the number you had

on your dice. If he comes to a “compulsory transit”, move the surfer along the arrow until the next website.

5 Every time the surfer comes to a website, that website’s owner gains 1 point. Write

it down under the name written on the paper.

Repeat 4 and 5 30 times. The winner is the player who did a better score summing all the points gained with the different chosen websites. If you have the same score, the winner is the player who has more points on a single website.

In the previous pages, we have seen how search engines work. Now, let’s play with what we have learnt. You can play by yourself or with other 2 people. Ask an adult to help you cut out the sur-fer’s string you find below. Put it on the other page where you have the graph, and read the rules!

Which is the website with the highest score? And the second one? As you realized, PLaNCK!’s website is the one where it is easier to arrive, because 5 websites have a link to it. But also grandma Rose’s Blog and the University of Padua’s website have a lot of points, because they are the only ones reachable through PLaNCK!’s website. If we used the PageRank algorithm, the order would be:

1. www.planck-magazine.it2. www.unipd.it3. www.nonnarosa.plk4. www.notizie.plk5. www.bufale.plk6. www.max.plk7. www.fumetti.plk8. www.tech.plk9. www.marie.plk10. www.info.plk

So the most important website is the one where it is easier to arrive!

What happened?

Difficulty level:

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play with your friends!