The glass of an electric light bulb is not much thicker than the paper of this page, yet it can endure a strong grip when you push into the bulb socket. The explanation for this lies mainly in the bulb's shape, which is based on the eggshell principle.
Why don't we get crushed even when a hen sits on the eggs to incubate them. The secrets lies on the egg's shape. That "egg shape" provides strength in the structure, which should withstand the all-round pressure even with a thin shell. If the shell is too thick, the offspring inside would not be able to peck its way out.
This characteristic was imitated when light bulbs were designed. The round and curved profile of the bulb allows the force you apply to be transmitted in all directions away from the point of contact. This makes the force to be distributed over a wide area and no excessive stress being set up at any one point.
Topic: Physics. http://en.allexperts.com/q/Physics-1358/bulb.htm
The Reader's Digest Assoc. Ltd., How Is It Done?. Why paper thin bulbs are so strong? (London:The Reader's Digest Assoc. Ltd.,1990),11.
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