not using gelatin but a real key lime pie.
Because they taste different.
limes have no pits and are sweeter
Lemons are more sour and acidic than Limes. It does make a difference as far as taste between one and the other.
Where you can use limes you can use lemons but they have a totally different taste. I use limes a lot more than lemons but thats personal preference. If I had no limes I would often use lemons.
