The answer is very simple:

I add a few eye drops into the mascara tube and mix the solution together using the mascara wand, the mascara will last a few extra uses to allow you a chance to buy another one! 

Eye drops are a useful thing to carry in your make-up bag as they calm your eyes if you have an eyelash in your eye or if you have been wearing contact lenses for a long period of time. I use a brand called Blink which come in single use portions available in most pharmacies.