[1] Bumping occurs when a liquid is heated or has its pressure reduced very rapidly, typically in smooth, clean glassware.
This rapid expulsion of boiling liquid poses a serious hazard to others and oneself in the lab.
The most common way of preventing bumping is by adding one or two boiling chips to the reaction vessel.
In addition, heating test tubes should never be pointed towards any person, just in case bumping does occur.
[2] A sealed capillary tube can also be placed in a boiling solution to provide a nucleation site, reducing the bumping risk and allowing its easy removal from a system.