Simplify the following text for a Grade 4 native English-speaking person.
Original: The other way that trading is made instrumental in gameplay is that many PokÃ mon , like Haunter and Machoke , can not evolve into their final forms without being traded . Gold and Silver built upon this idea with the advent of holding items ; some PokÃ mon only evolve when given certain items to be held and then trading them .
Simplified: Some PokÃ mon only evolve if they are traded to a friend .