I saw this idea on someone else's blog a long time ago and thought I would share. I gathered a bunch of little rocks from down by the river and painted numbers on them.
On some rocks, I painted an addition sign on one side and a subtraction sign on the other. Then there are also rocks with "equals" signs. All of the rocks that have numbers have one number on one side and another number on the other side. I painted all the odd numbers in blue and the even numbers in red.
I just used our washable tempera paint. I had to give them a couple coats. When they were completely dry, I sealed them with Mod Podge.
They are great for practicing number families. For example, Primus made the number sentence, "8+12=20." Well, then we took the 20, moved it in front of the 8, and adjusted the symbol rocks so the new sentence was, "20-8=12." The 8 and the 12 could also be switched.