be like quicksilver

"Quicksilver," another name for the metallic liquid element mercury, is a translation of the Old English term cwicseolfor, which means "living silver" and referred to mercury's ability to flow quickly.

1. To move or change very quickly. Sure, the boss is 100% behind this project now, but his mind is like quicksilver—who knows how he'll feel about it tomorrow. I can't believe the year is already almost over. Time seems to be more and more like quicksilver as I get older. Wow, my little niece is just like quicksilver—she's a baby one day and a little kid the next.

2. To be very hard to keep or maintain. Customers in this market are often fickle, so profits can sometimes be a bit like quicksilver. Don't get too attached to success, my boy, it's like quicksilver. A career in the major leagues is like quicksilver for these players on the fringes of the roster.