When two blocks of ice are pressed against each other then they stick together (coalesce) because

Explanation

i. When two ice pieces are pressed against each other for a while, their melting point lowers below `0^(@)C`.
ii . This causes partial conversion of solid surfaces of ice pieces into water.
iii. This results into merging the surfaces of two pieces into one.
iv. As soon as the pressure is removed melting point is restored to `0^@C`causing re-freezing of water into ice.
Hence, due to regelation, when two small pieces of ice are taken and pressed against each other for a while, they stick to each other.