Like all materials, sugar and water are both made of very small particles.  What happens when you add the sugar particles to the water particles?  Imagine your school football team running onto the pitch - they are very visible! 

Now imagine they have won the match and all the fans run onto the pitch in excitement!  If 10 or 20 fans surround each player, you cannot see the players any more.  They are separated from each other and surrounded by the fans. 

In the same way, when you add sugar to water, the water particles are attracted to the sugar particles ( like magnets).  Each sugar particle is surrounded by lots of water particles.  Therefore, each sugar particle is spread out and separated from the other sugar particles, so the white crystals of sugar appear to disappear.

We say that the sugar is soluble in water and so has dissolved in the water.  The sugar (the solute) has dissolved in the water (the solvent) to form a solution.

If the water is hot, the particles spread faster because they have more energy to move - imagine this time the football players and fans jumping up and down, doing cartwheels and running faster than before - they are going to spread out more quickly aren't they?