  The idea that Lorentz-symmetry in momentum space could be modified but still
remain observer-independent has received quite some attention in the recent
years. This modified Lorentz-symmetry, which has been argued to arise in Loop
Quantum Gravity, is being used as a phenomenological model to test possibly
observable effects of quantum gravity. The most pressing problem in these
models is the treatment of multi-particle states, known as the 'soccer-ball
problem'. This article briefly reviews the problem and the status of existing
solution attempts.
