This one is silly but we’ll see if it helps me remember the difference between these two words.

Bolder is to be very outgoing.
Boulder is a big stone.
Maybe this will help me remember. When you toss a boUlder into soft dirt the weight can presses the rock deep into the sand thus making a crater or U shape in the ground. That might remind me that boUlder.

Got a better idea? Please share it in the comment section of this blog.