Place a small mirror in a clear glass of water. Put the glass on a sunny windowsill. Position the mirror in the glass so the sun will hit it and cause a rainbow to shine on the wall. Point out the rainbow to your child, then move the glass to make a rainbow somewhere else.
Tape three or four different colored crayons in a straight line. Show your child how to draw a rainbow with one stroke.
How can I make an easy costume?