How do I explain occasionally to ESL?

You could draw a certain item over and over and occasionally throw in another. Like, 4 triangles in a row and the a circle, then 6 triangles then a circle, then 5 triangles and then a circle. The circle comes up occasionally. You could relate it to math by saying less than 50%. They should understand that 0% < occasionally > 50%. Use the words "every now and then", or "once in a while", or "not often". The antonym obviously would be often, so with the circles and triangles you could say that triangles happen often and circles ocionally.