Why is Rosencrantz referred to as a "sponge?"

In Act IV, Scene II of Hamlet, Rosencrantz and Guildenstern, two childhood friends of Hamlet, are working with King Claudius against the Prince. They come to Hamlet asking for Polonius's body:

Rosencrantz:

Tell us where [Polonius's body] 'tis, that we may take it thence
And bear it to the chapel.

Hamlet

Do not believe it.

Rosencrantz

Believe what?

Hamlet

That I can keep your counsel and not mine own.
Besides, to be demanded of a sponge! what
replication should be made by the son of a king?

Rosencrantz

Take you me for a sponge, my lord?

Hamlet

Ay, sir, that soaks up the king's countenance, his rewards, his authorities. But such officers do the king best service in the end: he keeps them, like an ape, in the corner of his jaw; first mouthed, to be last swallowed: when he needs what you have gleaned, it is but squeezing you, and, sponge, you
shall be dry again.

By referring to Rosencrantz as a sponge, Hamlet makes it clear that he is suspicious of his friend. He believes that Rosencrantz is an opportunist, and is eager to ally himself with King Claudius. Like a sponge, he wants to soak up all the benefits of being in subservience to someone with great power. Hamlet suggests, however, that once Claudius is through with him Rosencrantz will simply be "wrung out to dry."