Sometimes we can't always clean our brushes immediately after painting with acrylics. Here's how to make cleanup much easier when you do get around to it: add a small amount of Murphy Oil Soap*to the water you're resting your brushes in. (A teaspoon should do it.) You'll find the paint washes out of the bristles more readily, making cleanup go much faster.
*I'm in no way connected to the makers of this product and receive no compensation from them. They don't even know I exist. Murphy's happens to be the best brush cleaner I have ever come across, and is indispensable in my studio .