Think about it... Your great grandpa who lived centuries ago and who is still known for the great inventions he made in the past wrote you a letter! Ali Qushji the space engineer, Al-Biruni the famous mathematician, Evliya Chelebi the traveller, Al-Farabi the second teacher in the world, Hezarfen Ahmed Chelebi the first aircraft engineer and Ibn Khaldun the most important historian and philosopher of the world wrote you a very special letter. It is very exciting, isn't it? When you read these letters, you will both travel to the times your grandpas lived in and learn about their wonderful works. Come on, kids! Now it is time to read the letters from our "Wise Grandpas"...