The explanation..!?

As you can see, when the weights are placed onto the plasticine, there is force (orange lines) acting downwards on the plasticine. The mass of the weights in addition with the force causes the plasticine to expand as it stretches downwards. This causes the length of the plasticine to increase but it will also cause the thickness of the plasticine to decrease. These two points that we’ve mentioned has an effect to the strength of the plasticine. As a result, the strip of plasticine becomes too thin and weak to withstand the weights and in the end breaks in half, causing both the plasticine strip and the weights to fall.