There is nothing simpler than a test of strength to prove - or disprove - a product's durability. Zagg, a United States-based manufacturer of protective coverings for mobile phones and portable media players, went to a gym to do just that. Its product, a lightweight, see-through anti-scratch adhesive film, is based on technology originally used by the military to protect helicopter rotor blades from dust, dirt and debris. On YouTube (keywords: 'invisible shield strength test'), the product proves it can easily handle up to 454.5kg of stacked weights. Your mobile phone may snap in two under all that weight, but it certainly will not get scratched.