How is it possible to lie without harm on a bed of nails?

We know that pressure is a force divided by area over which the force is exerted. When a person lies on a bed of nails, the force equal to the weight of that person acts on the surface of the bed containing nails. Here, the weight is distributed over hundreds of nails that makes the pressure at the tip of each nail safely small. If the same person stands on his feet, he will surely feel the sharpness of the nails because in this case the area of  contact is very small. So, the weight will be distributed over a few nails and hence the pressure will be relatively large.   

What is the reason of cooler air coming out of a bursting tire?

We know from the ideal gas law that pressure increases proportionally with the increase of temperature when volume is held constant for a given mass. As the air inside the tire is in compressed state, the temperature inside the tire is high. When the tire bursts, air inside the tire comes out very fast causing a sudden expansion and decrease in temperature. Therefore, the air is relatively cooler than the surroundings. Blowout of tire occurs when the location of tearing of the tire is incapable of containing the pressurized air and the escaping air causes further tear of the tire.