Distinct excursion intervals of a Brownian motion (that correspond
to a fixed level) have no common endpoints. What is the situation for distinct
excursion sets of a Brownian sheet? These sets are termed Brownian bubbles in
the literature, and this paper examines how bubbles from fixed or random levels
come into contact with each other, by examining whether or not the Brownian
sheet restricted to a specific type of curve can have a point of increase. At
random levels, we show that points of increase can occur along horizontal
lines, while at fixed levels, such a point of increase can occur at the corner
of a broken line segment with a right-angle. In addition, the Hausdorff
dimension of the set of points with this last property is shown to be 1/2
a.s.