These are eco-friendly modeling clays, harder than others, reminiscent of the old modeling clays that were available in our country in the past. The advantage is that they do not have the petroleum smell that those had. Also, being slightly harder, they allow for the creation of more durable constructions for children to play with, and they allow for creations with different colors without easily mixing. Finally, I have the feeling that they somewhat better exercise the fingers of children and their fine finger movements. Personally, I prefer them compared to the more malleable, softer ones on the market.