'Thoppikaranam' the special form of obeisance offered only to Lord Vinayaka, is the corrupt form of the Sanskrit word 'Dorbih Karanam Griheetvaa'. One of the celestials is said to have offered this form of salutation to Ganapathi in order to make him laugh and throw out the discus of Lord Vishnu he had swallowed mischievously. After all Sri Vishnu was his uncle and the nephew could afford to take some liberties with Him. Sice then, this form of salutation, exclusively meant for Sri Vinayaka has come to stay.

Dorbih Karanam Origin

One more version is, there was a demon Gajamukaasura (he was elephant-faced and hence this name), who, by virtue of the tremendous powers he had earned as a result of his unmatched austerities, harassed the celestials intolerably. He made Indra, the chief of celestials to bend on his kness, catching his ears in a criss-cross fashion with his hands, squat and rise a 1008 times every morning. All the other celestials also were forced to offer this humiliating form of salutation to the demon.

'Mooshika Vaahana...'

Eventually, in order to free the gods from the tyranny of Gajamukaasura, Lord Vinayaka fought with him, defeated him, and converted him into the form of a 'Mooshika' (a bandicoot, a large rat) and used him, thenceforth, as His vehicle. In sheer grateful and reverential appreciation of their liberation from the clutches of the demon by Lord Ganapathi, the celestials offered to pay this special form of salutation to the Lord from then on. The phrase, 'Dhorbi Karanam Griheetvaa' literally describes this salutation only - 'catching the ears with hands across and squatting and rising before the Lord in reverential salutations for a prescribed number of times - 10, 28, 108,..etc. depending on one's inclinations.

Cuffing the Head

In addition, the devotee also knocks on his head, with his knuckles, with some force, a number of times (cuffing the head), as a mark of humble reverence. No other God is offered such exclusive form of worship. 

Interesting Interlude Between Brothers

Sivaprakasa Swamigal narrates a humorous interlude between Lord Siva and His two sons Vinayaka and Muruga. The elder son, the elephant-faced one, complained to his father thus: 'Father! Brother Muruga pinched my ears painfully. The Divine Father summoned the younger son and asked for an explanation for his disrespectful behaviour towards his elder brother. The younger one protested: 'But, father, it was 'Anna' (elder brother) who first provoked me by counting my twelve eyes, one by one, in my six faces mockingly! If Ganesa was an elephant-faced son, then Muruga was Shanmukha, the six-faced one. Now Siva asked Ganesa : 'Son, was that not bad on your part?'

Measuring the trunk: Vinayaka retaliated: "But, 'Thambi' (younger brother) started it all first by measuring my trunk mischievously and kept silent. Lord Siva could not suppress his own smiles and looked at His Consort Parvati as if to say, "Look at your children, Uma!" But, if Muruga got the better of his elder brother this time, the elder one  'Gajapathi' taught a lesson to His younger brother Shanmukha soon! That is an interesting story, of course!