Explain Hamlet’s logic in calling Rosencrantz and Guildenstern “sponges.”

Hamlet says that like a sponge, Rosencrantz and Guildenstern soak up information for the king and then the King squeezes it out of them. Then they are ready to be used again,

"When he needs what you have gleaned, it is but squeezing you and, sponge, you shall be dry again."