For the eager youngster, head-butting may appear simple, but mastery of this skill requires timing, cooperation, and awareness of basic principles of geometry…
For example, it is important that the participants align themselves within a single two-dimensional space, lest they accidentally veer into a perpendicular “side butt.”
It is also important to concentrate completely upon the maneuver, and not allow oneself to become distracted by other goings on.
But with practice and teamwork, head-butting will become as easy as falling off a log (unless you happen to pick a log as your practice space).
Via Martin Pettitt.