Help! I did a laboratory on how to separate a mixture of sand and salt from each other. To do this i decided to add water and dissolve the salt...

The explanation is probably a lot simpler.  You correctly added water to the sand and salt mixture.  The salt dissolves in the water but the sand of course does not.  The sand is then removed by pouring the mixture through a sieve.  The saltwater solution passes through the sieve while the sand is trapped and separated.  Then, the salt is isolated by boiling away the water to leave behind solid salt.

The boiling action of the water is inherently kinetic due to excess thermal energy from the heat source (stove).  A rolling boil splashes saltwater solution around.  If any splashes out of the bowl, then when it contacts the hot stove surface the water will quickly evaporate and leave solid salt residue behind.  If you are using a kitchen type bowl to heat the water in then it has a large opening at the top which is perfect for boiling water to splash out of.  Next time, boil the saltwater solution in a beaker or similar piece of glassware.  They have a much narrower mouth at the top, thus keeping the salt water better contained during the boiling process.