Contact solution
We only get one pair of eyes, to last us our entire lifetime, and they’re one of the most sensitive parts of our bodies. We need to look after them if we want them to see (pun intended) us through to old age. So, if you wear contacts, then it’s imperative you change your solution every 3 months.
This is because an invisible layer of film can form on the lens – a biofilm. The biofilm protects the lenses from the solution (which is the opposite of what you want to happen), and so can increase your risk of infection. Doesn’t really seem worth saving a few dollars, does it?