Like millions of other people, Palomar College professor Pat Hahn is a smartphone user. A few weeks ago, he gave his Apple iPhone to his 5-year-old grandson, Brayden, to play the popular game "Angry Birds."But a simple 99 cent purchase turned into a bill of more than $80."At first, I was shocked; [I] thought somebody got my credit card," said Hahn.Hahn soon learned Brayden unknowingly bought what is called an "in-app purchase" while playing on the phone. The feature is a pop-up within a game that offers extra features and costs real money.Since Apple already had Hahn's credit card information, Brayden inadvertently bought it with a simple push of the screen."At 70 percent off, for $79.99, it's apparently weapons and you could do all kinds of things with it," saidWhile some would consider it deceptive, in-app purchases are a lot more common than people think."No game is really free," said San Diego State University professor Murray Jennex.Jennex told 10News the practice has been around for a while and will continue to grow."This is a whole new economy that's developing in virtual tools and virtual money that turns out to be real money," said Jennex.A study by PlaySpan, an industry group which analyzes video game behavior, showed 31 percent of gamers have purchased virtual content.However, the problem is when young children who use smartphones don't understand video game money versus real cash.Hahn learned there is a way to disable in-app purchases, but it is deep within his iPhone settings under "restrictions.""I think it's really sneaky, it really is," said Hahn.Computer experts say all smartphone users can really do is keep a close eye on who uses their phone in order to make sure they are not paying a big price for free fun.