When we refer to someone as "slippery as an eel", we're often implying that they're elusive, elusive, and difficult to pin down. But language is a nuanced thing, and sometimes a different turn of phrase or a more fitting metaphor can be useful. If we wanted to describe someone as particularly hard to hold on to, we might say they were "as elusive as quicksilver". Alternatively, if we wanted to get across the idea of someone who is persistently vague or hard to understand, we might say they were "as evasive as a slippery politician". In either case, the underlying idea is the same, but a different comparison can add some extra nuance.