There is a huge amount of plastic trash floating in the ocean, which endangers wildlife that eats or gets tangled in it. Reducing the amount of plastic trash in the ocean doesn't seem that hard; people just need to use less plastic, such as packaging, drinking straws and plastic bags. But it can be very hard to break people's habits.

In 2002, Ireland made a simple change: they started charging a 15 cent tax on plastic bags at stores, which used to be given away free of charge. They hoped having to pay for bags would make shoppers bring their own reusable bags from home, so that there would be less plastic litter along the coasts and in the countryside. 

And it worked! After the tax was put in place, plastic bag use went down 90 percent, and the number of areas that were mostly litter-free went up 50 percent. Remarkably, it was also hugely popular with shoppers. "We are not aware of another tax that induces such an enthusiasm and affection from those who are liable to pay it," wrote the researchers. Other states and countries have followed suit and implemented similar plastic bag taxes.