But if you really want to then check out trading and how to do it and how it works, 100$ is a start which can make profit although it will go slow.
No one can earn huge profits right after starting to do something, so if someone has a capital like that he should probably try trading with it and if he can manage his trades in a good way he will probably increase his capital with the time.
Exactly trading is the best way to grow his funds, no doubt $100 is not a big amount to invest but you can expect certain profits from trading which will eventually grow his investment.