Theory

Many verbs in English are given an ING ending.
This sometimes causes their spelling to change.
Example:
PLAY becomes PLAYING with no change, but SIT becomes SITTING where the last T is doubled

There are some simple rules for these spelling changes, but there are always exceptions to rules.
For most words, adding ING doesn't change the original word.
Example:
MAIL + ING gives MAILING.

If a word ends in Y, keep the Y when adding ING.
Example:
PLAY + ING gives PLAYING.

If a word ends in E, drop the E when adding ING.
Example:
RIDE + ING gives RIDING.

If a word is short and ends in a consonant, double the consonant.
Example:
SIT + ING gives SITTING.

If a word ends in double EE, do not drop the second E.
Example:
SEE + ING gives SEEING.

If a word ends in IE, change the IE to Y then add ING.
Example:
LIE + ING gives LYING.

If a word ends in YE, add ING.
Example:
DYE + ING gives DYEING.

If a word ends in a silent letter, add ING.
Example:
CLIMB + ING gives CLIMBING.

If a word ends in I, add ING.
Example:
SKI + ING gives SKIING.