Why can't absolute zero (0 K ) be reached ?

I don't understand why 0 Kelvins can't be reached ? I see that the world record for absence of heat is 100 pico kelvins. In certain astronomical instances, as low as 1 k has been observed. Is there a physical reason why 0 k can't be achieved ? And can you explain it in lay terms that most would understand ? Very curious.....