Drawing depicting possible scenarios of galaxy evolution in the main sequence framework. Scenario 1 (left panel): external gas inflow triggers a compaction event. The gas is funneled to the center of the galaxy and it moves to the upper bound of the MS. This phase is characterized by a compact core with a high f_gas, high SFR, and low τ_dep, compared to normal MS SFGs at a fixed stellar mass and redshift. Central gas depletion leads to inside-out quenching and the galaxy moves to the lower bound of the MS. This implies that when MS SFGs have low f_gas, they exhibit a more extended star-forming region in comparison to the previous phase. Scenario 2 (middle panel): external gas supply from gas-rich mergers. The gas is funneled to the center of collision, enhances SFR, and the galaxy moves well above the MS. As the gas reservoir is consumed, the SFR declines and the galaxy crosses, incidentally, the MS. Scenario 3 (right panel): angular momentum loss is driven externally (accretion through mergers or counter-rotating streams) or internally (clump migration). The gas is funneled to the center of the galaxy in either case (represented by a merger in the panel), enhances SFR (although not necessarily directly or well above the MS in all cases). As the gas reservoir is consumed the star-forming region becomes more compact sustaining the SFR (outside-in gas consumption). In all three scenarios, the assumption is that at high masses the hot dark matter halo prevents further gas inflow and the galaxy eventually quenches.