This is very true for leopards geckos under 6 inches in length. However, many breeders prefer sand for their adult geckos who are the appropriate size. It is my understanding that ingesting very small amounts of sand and/or other particles can actually be beneficial for your geckos, as it aids in mechanical digestion of crickets, mealworms, etc., which is said to allow for improved availability of nutrients. I have even read that some breeders even add a small amount of sand to the feeding dish or calcium dish for this very reason. I think of it like this, geckos lived for many years in nature, outside the paper-bottomed, rubbermaid shoes boxes stacked in breeding racks, and it is likely that they frequently ingested rocks, sand, dirt, etc while consuming their prey in the wild. In my opinion, the idea that sand is terrible and to be avoided in every situation is in part spread by pet stores trying to sell their calcium substrate sand substitute (which, by the way, can also cause impaction in geckos smaller than 6 inches in length). Unfortunately, the calcium from this source is very minimally, if at all, disolved and/or absorbed by the gecko. Many breeders use newspaper or paper towel liner for their housing because easily removed, discarded, and replaced when regularly caring for large quantities of animals. More importantly, it is cheap.